AbstractThe predominance ofxover λ light chain expression in mice can either reflect the probability of rearrangement of the relevant locus or be the result of antigen‐driven clonal expansion. To discriminate between these two possibilities we determined, by limiting dilution analysis, the frequencies ofx‐ and λ‐producing cells in B lymphocytes generatedin vitrofrom bone marrow pre‐B cells. The frequencies obtained in these cultures are not significantly different from those obtained with mature spleen cells. In addition, Southern blot analysis of bone marrow‐derived and splenic cell DNA revealed that in both populations the extent of B lymphocytes having undergone λ1 gene rearrangement does not exceed 4%. These results, therefore, establish that in the mouse the low level of λ light chain expression directly reflects the probability of rearrangement of the r