Variants of four hybridoma lines secreting antibodies specific for either trinitrophenyl (TNP) or sheep erythrocytes (SRBC) were isolated which have lost either the specific heavy (H) chain or the specific light (L) chain. They were fused to lipopolysaccharide-stimulated mouse spleen cells and the resulting secondary hybridomas were screened for the restoration of the original antibody specificity. Antibody activity was 37 times more frequently restored with fusion lines donating H chain than with those donating L chain. We obtained a variety of different (spleen cell derived) L chains in association with one H chain and one specificity. We found that those L chains originally associated with a given H chain were rescued most often. Their frequencies were 1:74 and 1:490 for an anti-SRBC- and an anti-TNP-restoring kappa L chain, respectively. Two most commonly observed V-J kappa combinations in one anti-SRBC complementation group were detected with a 5- and 30-fold reduced frequency in two anti-TNP groups, indicating somatic diversification of kappa chains. It is shown that the Sp7/HK line resumes anti-TNP activity with a mutated, non-germline lambda 1 chain, which was found in 30% of lambda 1-expressing hybridomas.