A cDNA library prepared from a human fetal liver of the first trimester of gestation was screened with Ig C mu, C gamma, C kappa and C lambda probes. Ten heavy chain clones were isolated and characterized by restriction mapping and partial sequencing. The absence of Ig light chain clone and the presence of pre-B-specific lambda-like transcripts suggest that the immune compartment of this cDNA library was mostly derived from pre-B cells. Three transcripts of mu, gamma 2 and gamma 4 isotypes contained a V-D-J-C region with an open reading frame and used members of the VHIV, VHIII and VHI families, respectively. Seven clones were derived from sterile transcripts, one C mu and six C gamma. In addition to C mu exons, the sterile mu transcript contained the 5' flanking germline region. By contrast, the gamma sterile transcripts used a 5' sequence that was spliced from the I gamma 1 region onto the first C gamma 1 exon. In addition several of these transcripts were derived from alternative splicing. The simultaneous expression of both sterile and functional gamma transcripts suggests that the switch mechanism operates in normal fetal liver very early in ontogeny.