In early steps of B cell differentiation, mu chains are transiently expressed in association with a surrogate light chain (psi L) composed of the lambda-like and VpreB monomorphic polypeptides, thus forming a putative preB receptor. Using a monoclonal anti-VpreB antibody, preB cells were isolated from two adult human bone marrow samples and their VDJ repertoire analyzed at the transcription level. All VH families were identified and further analysis focused on VH3 sequence analysis of 37 distinct VDJ cDNA clones. The VH3 genes expressed in the two bone marrow samples were also encountered in fetal liver and adult peripheral blood lymphocytes with a roughly similar contribution of 3.30, 3.23, 3.9 and 3.53. The characteristic features of the preB repertoire as compared to the activation B repertoire include the quasi absence of somatic mutations, limited N diversity and a shorter third complementarity-determining region (CDR3). It also significantly differs from the fetal repertoire, which makes higher usage of DQ52 and has CDR3 of even shorter lengths. The almost constant presence of glycine residues in the CDR3 and predominance of JH4 with a low level of DQ52 DH usage, suggest that preB cell clones are submitted to an initial selective pressure which should be antigen independent. The bona fide heavy chains would be merely selected for their ability to interact with the surrogate light chains, thus shaping the repertoire that will be co-expressed with immunoglobulin light chains in IgM molecules.