Human HB4C5 hybridoma cells produce a lung cancer-specific human monoclonal antibody that possesses a lambda light chain on which a N-linked carbohydrate chain is attached at the first complementary determining region. Up to six light chains of various sizes are secreted by the original and concanavalin A (ConA)-resistant HB4C5 clones. Two of these six light chains are derived directly from the HB4C5 original and are 30 and 32 kDa in size. The structural difference between these two light chains has been shown to be a N-linked carbohydrate located at a single site. The other four variants, which are derived from ConA-resistant variants, have light chain sizes ranging from about 26 to 29 kDa. No changes in the secretory forms were observed when glycosylation was inhibited by the addition of tunicamycin, indicating that the heterogeneous light chain forms produced by these ConA-resistant variants resulted from the size differences of core polypeptides and not from N-glycosylation. Sequence analysis from one of the variant light chains revealed that its V lambda gene segment differs markedly from identified human V lambda gene segments (at most, 60% homology with known human V lambda subgroups) and, furthermore, the variant light chain shares 80% homology with the rabbit V lambda gene. In addition, the antigen binding ability of these variant antibodies changed significantly in both specificity and affinity. These results suggest that heterogeneous light chains expression by ConA-resistant variants may provide a new way for developing an antigen-specific human monoclonal antibody repertoire.