Mice transgenic for the lambda 2 light chain of MOPC315 were established. In newborn transgenics (TG), lambda 2 was the only light chain found on B cells. However, by day 21, lambda 2 high kappa low as well as lambda 2 low kappa high double expresser populations were emerging. lambda 2 was found on an increased fraction of serum immunoglobulins (Ig), this fraction declined with age. Correspondingly, kappa and lambda 1 expression was suppressed in young mice but increased with age. In adult mice kappa or lambda 1 were often co-expressed with lambda 2 in single serum Ig molecules. Most B cell hybridomas from and adult TG secreted lambda 2,kappa mixed molecules and had rearranged their kappa chain genes. One lambda 2,lambda 1 hybridoma and even a lambda 2,kappa,lambda 1 hybridoma were also found. In conclusion, isotypic exclusion in lambda 2 TG is complete in newborns but becomes increasingly leaky with age. Antigen probably expands the lambda 2 low kappa high B cell population; this population is most likely the major source of serum Ig in adult lambda 2 TG mice. In contrast, the lambda 2 high kappa low population, a major fraction of which is CD5+ Mu low delta low, appears only infrequently to develop into antibody-secreting plasma cells.