CD40 ligand (CD40L) and IL-4 are sufficient to induce resting murine B cells to divide and switch isotypes from IgM and IgD to IgG1 and IgE. Tracking of cell division following (5- and 6) carboxyfluorescein diacetate succinimidyl ester (CFSE) labeling revealed that B cells expressed IgG1 after three cell divisions, and IgE after five. The probability of isotype switching at each division was independent of both time after stimulation and of the dose of CD40L. IL-4 concentration regulated the number of divisions that preceded isotype switching. Loss of surface IgM and IgD was also related to cell division and appeared to be differentially regulated. B cell proliferation was typically asynchronous with the proportion of cells in consecutive divisions being markedly affected by the concentration of CD40L and IL-4. Simultaneous (5-bromo)-2'-deoxyuridine labeling and CFSE staining revealed that B cells in each division cycle were dividing at the same rate. Therefore, division cycle asynchrony resulted from dose-dependent variation in the time taken to enter the first division cycle. These results suggest that T-dependent B cell expansion is linked to predictable functional changes that may, in part, explain why IgE is produced in response to prolonged antigenic stimulation.