The possibility that isotype switching in B cells may be affected by engagement of the CD45 molecule on B cells has been investigated in microcultures containing limiting numbers of B cells and nonlimiting numbers of both alloreactive Th cells and purified dendritic cells (DC). Addition of Abs to the B cell-specific isoform, B220, to the microcultures leads to an increase in the proportion of B cell clones that secrete secondary Ig isotypes. In the presence of anti-CD45 Ab, microculture wells show a 39% frequency of secondary isotypes (560/1440) compared with a 11% frequency in control microcultures (89/780). Cross-linking appears to enhance this effect. Even in cultures of B cells and DC without T cells, addition of anti-B220 induces isotype switching in a significant number of microwells. Cross-linking and capping B220 molecules results in co-capping of surface Ig and MHC class II molecules. The results suggest that signal transduction through the CD45 molecule may affect pathways involved in isotype switching.