The emergence of militant organizations or new political movements always seems rather strange and accidental : why do a number of movements generate particularly strong commitments and find numerous relays, whereas others find it difficult to make themselves known and widen their audience beyond the initial circle of their founders ? This thesis about SOS-Racism – a French antiracist movement founded in 1984 – originated from a question about the conditions of success in politics. Why, at that time, did a rather marginal group of students, gathering round Julien Dray, wish to found a new antiracist organization, a field they knew nothing about ? How were they able to reach such a quick success and what were the causes of their subsequent relative decline ? Moreover how can we explain that their mobilization took the form – discredited some years before – of an « apolitical » movement ? This thesis seeks to answer these questions by trying to link an interrogation about the political entrepreneurs' characteristics and motivations and an effort to understand the objective conditions of the success of their entreprise. First of all we studied the conditions in which SOS-Racism was founded. We had to understand how former extreme left militant students who considered they belonged to the left wing of the Socialist Party, could have imagined an organization labelled « apolitical ». We also had to explain how the founders of SOS had chosen to start their new association at that very moment though they already were members of several other partisan associations (PS-Socialist Party/MJS- Movement of Socialist Youth/UNEF-ID-National Union of the French students-independent and democratic). This thesis, originating from biographical interviews with the founder members (about twenty, plus fifty or so interviews with the officials and the militants of the organization) tends to show that the reasons leading to the creation of SOS-Racism came more from the minority position its founders had in the various associations they belonged to and from the opportunities for new placements they could have in the Socialist Party, rather than from a planned antiracist strategy. So the birth of this new organization mainly depends on the militant and professional constraints proper to its future leaders. It has been the main way to accumulate political resources for its founders, resources which will be later convertedinto the creation of a new political current, the NES (New Socialist School), Julien Dray's embryonic current within the Socialist Party, However, the analysis of the specific logic leading to the creation of a new antiracist organization couldn't explain the causes of its success. So, this thesis tries to understand the reasons why a number of newspapers had written so many articles and reports about an association with few militants and poor means of action, at that time. We had to find out if the form initially adopted by SOS-Racism and more particularly the efforts its leaders made to appear politically neutral but also their great efforts to show off their militants' spontaneity and youth, could have been a factor favouring the surprising level of interest the journalists soon showed for it. The setting up of a data bank for the articles published about the association in the national papers and the thirty or so interviews we had with the journalists who had written about SOS allowed us to determine the logic of the fluctuations in the journalists' judgements on SOS. The origin of this success was the adaptation of this politically neutral attitude favoured by the new political organization to the new needs of the press. As a matter of fact an « apolitical » antiracist organization allowed then the press associated with the left (Libération, le Matin de Paris, le Nouvel Observateur) to hold ideological points of view appearing as progressive ; at the same time, their strategy was to move away from their commitment especially by giving up their usual support to the government considered, at that time, as the cause of the dropping circulation figures of the left press. On the contrary, when the public image of SOS-Racism is more and more linked with the Socialist Party, through Julien Dray, its founder, the support the journalists give it will get weaker and weaker; it will be quite useful to them, though, when it is called into question, later on. We may say that the relative decline of SOS will originate in the reversal of opinion in the left press when the journalists' support to the movement doesn't serve their professional interests any longer. Yet, to account for the structural conditions of emergence of an enterprise of « apolitical » mobilization about racism, it remained to point out how this particular structure, which couldn't have been thought of in 1979 or in 1980 when the antiracist organizations as a whole opposed Raymond Barre's policy consisting in favouring the immigrants' return home, became in 1985 the very condition of the new movement's success. We had to understand through which political and social process the accession of the left to the government in 1981 had been able to change the nature and the content of the left parties' political offer but also cause the « neutralization » of the editorial line of the papers close to the left and the decreasing range of the political themes existing for the social actors. The decline in popularity of all that seems to be linked with the government and the left thus compelled the founders of SOS-Racism to give their new association an « apolitical » structure. Thus this thesis represents a contribution to the analysis of the social and political movements as well as the workings of the press. More generally, and apart from the particular case studied here, this work may help to understand the relationship which in the eighties united the various actors in the public sphere: political actors, journalists, economic experts, intellectuals, etc... By showing how the changes in the political offer of the left have been able to modify the ideological « atmosphere » of the eighties – that is to say the relative frequency of the use of political themes because of their unequal social efficiency – this thesis represents a contribution to the analysis of the conditions of the structuring of the public debate in France.