We have isolated pre-B- and B-cell clones after transformation by Epstein-Barr virus (EBV) of human fetal bone marrow cells between weeks 8 and 13 of gestation. These clones were characterized for immunoglobulin (Ig) chain synthesis, the status (rearranged versus germ line) of the heavy (H) chain, kappa, and lambda loci, and of the Ig mRNA transcripts by using specific variable (V), VH, V kappa, and V lambda probes covering the almost complete IgV repertoire. Mature B cells could already be identified in the 8-week-old bone marrow, with both kappa and lambda isotypes being present. Nevertheless, at this stage, most of the clones had Xhe characteristics of pre-B cells, as indicated by the presence of mu transcripts (either functional or sterile) in the absence of L chains. The kinetics of gene rearrangements were compatible with the classical scheme H----kappa----lambda. A rapid expansion of the expressed repertoire occurred between the weeks 8 and 11, with 95% of the EBV clones having the characteristics of mature B cells. V gene family usage was analyzed for the three loci and compared with the pattern of expression observed at 30 weeks of gestation and in adult clones. The "adult" pattern was rapidly acquired for the H and kappa loci, with the major subgroups being VH3 and V kappa 1. When the expression of the repertoire was "normalized," the pattern of V usage correlated fairly well with the estimated number of VH genes, but differed noticeably for the kappa chains, suggesting that the VH and V kappa repertoires are not regulated by similar processes.