The expression of membrane and secreted IgM was analyzed during mitogen-induced differentiation of the murine B cell lymphoma CH12. To characterize the Ig genes used by CH12, the nucleotide sequences of the variable gene segments (V mu and V kappa) were determined. The expressed V mu gene segment belongs to the VHII NPb-related family. The D (FL16.1a) and J (JH2) segments are the same as those used by the NP-specific hybridoma B1-8. The V kappa used by CH12 is almost identical to those used by the oxazolone-specific hybridomas NQ5.89.4 and NQ7.7.1. Treatment with lipopolysaccharide (LPS) induces up to 80% of CH12 cells to secrete IgM within 48 hr of culture. The steady state levels of secreted mu (mu s) and kappa mRNA increase four to fivefold over this period in cells stimulated with LPS compared with unstimulated cells. The kinetics are similar for both mRNA and parallel the increase in IgM secretion. EL-4 supernatants induce comparable changes in m mu s and kappa transcript levels. The simultaneous increase in m mu s and kappa transcripts suggests that coordinate control of RNA levels is used to increase the synthesis of secretory IgM during differentiation. The level of mRNA encoding the membrane form of mu (mu m) remains constant in stimulated cells and increases slightly in unstimulated cells. While the net rates of synthesis of membrane-bound mu-chains remain similar during LPS stimulation, the level of surface IgM on secreting cells is reduced three to fivefold. These observations suggest that the level of surface IgM expression during differentiation of CH12 is controlled largely by post-translational mechanisms. Our results demonstrate that the CH12 cell line regulates the expression of membrane and secreted IgM differently during its differentiation. The changes in IgM expression in CH12 parallel those occurring in normal B cells after mitogen or antigen challenge. Thus, the in vitro differentiation of CH12 is a good model for the analysis of late stages of B cell development.