Treatment of resting murine B lymphocytes with CD40 ligand (CD40L) and IL-4 induces proliferation and a switch in immunoglobulin (Ig) isotype surface expression from IgM and IgD to IgG1 and IgE. Using a fluorescent dye to enable cell sorting according to cell division cycle number, we have examined molecular events associated with B cell differentiation, namely, germ-line transcription and DNA recombination. Digestion-circularisation polymerase chain reaction experiments showed that DNA recombination leading to isotype switching from IgM to IgG1 surface expression is division-dependent and was first detected after B cells had divided three times. Similarly, DNA rearrangement involving the IgE switch region was detectable only after five division cycles. These division cycle numbers correlate with the numbers of divisions required before surface expression of the switched isotype [P.D. Hodgkin, J.-H. Lee, A.B. Lyons, J. Exp. Med. 184 (1996) 277-281]. RT-PCR analyses also revealed that germ-line transcripts for both IgG1 and IgE increased with division number suggesting a threshold expression level may be required for recombination to occur.