Hybridoma cell lines were transfected with plasmids containing either a rearranged lambda 1 or a rearranged lambda 2 mouse gene. The levels of lambda-chains synthesized by these transfectants were very low or undetectable. Activation of the expression of the lambda 2 gene was achieved artificially by deleting a portion of the region upstream of the promoter. Analogous deletions in the fragment containing the lambda 1 gene did not result in gene activation suggesting that the upstream sequences of lambda 1 and lambda 2 genes have diverged enough to allow differential regulation of their expression. However, both genes were activated by insertion, at a position upstream of the promoter, of a fragment containing the K-chain gene enhancer. These results suggest that the complete set of sequence elements that mediate lambda gene activation during normal B-cell differentiation are not all contained in the fragments of genomic DNA cloned so far, and thus, at least some of them must be located at a considerable distance from the promoter.