The judicial left, initiated in the early 1970s, is often a priori seen, because it is not analyzed, as the work of young activists of the extreme left who, while mourning their youthful ideals, penetrated into the professional world. The creation of the SAF, in 1973, forces us to break with this uniform point of view. This article concerns the first two generations of the SAF: the Communist founders and the lawyers of the extreme left who joined in the second half of the 1970s. Having brought to light the tensions between these generations of lawyer-activists in various affiliations or partisan orientations, the article shows how the daily coexistence in the profession and in the union in a phase of establishment contributed to limiting the intergenerational tensions. It brings out the weight of the sociability and the daily professional constraints as well as institutional politics in the dynamics of intergenerational relations within a union.