Southern blot analyses of immunoglobulin light chain gene rearrangements in human leukemias and myelomas indicated that lambda loci in kappa-producing cells are largely unrearranged while kappa loci in lambda producers are often rearranged and inactivated by rearrangements of the kappa-deleting element (KDE). For a systematic analysis of the regulation of light chain rearrangements during early B cell development in normal human B cells also considering functionality of the rearrangements, we used FACS-sorted single naive kappa- and lambda-expressing B cells from peripheral blood of healthy humans. V(kappa)J(kappa) and V(lambda)J(lambda) joints and rearrangements involving the KDE were amplified simultaneously from single cells and sequenced. Whereas only 2 - 3 % of kappa-expressing cells carry V(lambda)J(lambda) joints, nearly all lambda-expressing cells have rearranged kappa loci and indeed carry V(kappa)J(kappa) joints. The V(kappa)J(kappa) joints in lambda-expressing cells exhibit preferential J(kappa)4 and J(kappa)5 over J(kappa)1 and J(kappa)2 usage compared to kappa-expressing cells. Thirty percent of the V(kappa)J(kappa) joints in lambda producers are rearranged in-frame. These data indicate extensive sequential V(kappa)-J(kappa) rearrangements and inactivation of functional V(kappa)J(kappa) joints in lambda-expressing cells, presumably before V(lambda)J(lambda) joining.