The potential light chain repertoires expressed by IgM antibodies of fetal liver and adult spleen cells were compared. B cells from these sources were stimulated by the polyclonal mitogen bacterial lipopolysaccharide and after 3 days were fused to construct hybridomas. The light chains synthesized by these hybridomas were isolated by electrophoresis and their heterogeneity was examined by isoelectric focusing. A total of 109 BALB/c and 77 CBA/J hybridoma-derived light chains obtained from fetal liver and adult spleen cells were analyzed in this way. Fifty-three and 45 unique light chians were obtained by BALB/c and C57BL/6 hybridomas, respectively. Some of these were repeated more frequently than others, but no significant differences were seen in the heterogeneity of the light chain spectrotypes of fetal and adult CBA/J and BALB/cJ mice. Furthermore, the κ:λ ratio of light chains was the same in fetal liver as adult-derived hybridomas. These results show that the heterogeneity of light chains as determined by IEF and the κ:λ ratio of IgM immunoglobulins is established very early in ontogeny.