In this study BALB/c B cell precursors responsive to the T-independent (TI) type 2 (TI-2) antigen, dextran B1355S (DEX), and the T-dependent (TD) derivative, dextran-Limulus hemocyanin (DEX-Hy) were examined for isotype and idiotope expression using the splenic focus assay. The predominant isotype detected in the TI assay was IgM, while IgA was the predominant isotype expressed in the TD assay. There was also a fourfold increase in the number of foci secreting more than one isotype in the TD assay vs. the TI assay without an overall change in anti-DEX precursor frequency, suggesting that carrier-primed T cells enhance the expression of non-IgM isotypes possibly by increasing the frequency of isotype switching by individual B cell precursors. A panel of distinct monoclonal antiidiotype antibodies (MAIDs) was then used to examine idiotope expression by antibodies secreted in splenic foci responding to DEX and DEX-Hy. This analysis revealed considerable diversity in the idiotope profiles expressed by all isotypes tested. There appeared to be no differences in idiotope diversity among the various isotypes. A similar diversity of idiotope profiles was obtained from both TI and TD splenic foci, indicating that a comparable degree of diversity was associated with the antibodies generated by TI and TD precursors. Idiotype analysis of IgM-IgA-secreting foci with a panel of monoclonal antiidiotope antibodies revealed slight idiotypic differences between the two isotypes secreted in the same focus in about half the cases. These results suggest that somatic variation occurs during the antigen-driven maturation of B cell precursors, within the 15-d time frame of the splenic focus assay, and may be associated with isotype switching.