The expression of kappa and lambda light chains in surface immunoglobulin (sIg) molecules on B lymphocytes differentiating from murine pre-B cell clones in vitro was analyzed. The four pre-B cell clones used represent a very early pre-B cell stage. They have their heavy chain loci DJ rearranged and their light chain loci in germ-line configuration. In order to grow in vitro, these clones require contact with stromal cells and the stimulatory activity of interleukin (IL) 7. Upon removal of IL 7 from the cultures, these clones differentiate within 3 days into sIg+ B cells. Between 7% and 12% of IgM+ B cells could be detected in these cultures. The majority (78%-92%) of the IgM+ B cells co-expressed kappa light chains. The percentage of lambda light chain expressing B cells was below detectable level. Upon lipopolysaccharide (LPS) stimulation, the percentages of IgM+ B cells increased dramatically (from 32%-64%). The majority (91%-97%) of the IgM+ B cells express kappa chains, but a very small percentage (3.1%-5.0%) express lambda. A similarly high kappa/lambda ratio was found in 418 hybridomas prepared from these LPS-stimulated B cells (388 kappa+ and 30 lambda+). Thus, the high kappa/lambda ratio characteristic of the mouse peripheral B cell repertoire is already evident in the antigen-independent transition from pre-B to B cells.