The construction of social networks and their evolution in early adult years relate closely to processes of socialization. The authors draw on the results of a qualitative longitudinal survey launched in 1995 on a panel of young people questioned every three years. They analyse the impact of entry into professional life on the transformation of the respondents' social networks. Their findings show major changes not only in the numbers of individuals concerned but also in ways of building relationships on the basis of school and then professional frames, and on that of comparison between the school friend and the work colleague figure. The idea of an overall transformation of social relations in early adult life is confirmed.