The influence of immunization with (T,G)-A--L on the frequency and characteristics of [(125)I] (T,G)-A--L-binding cells (ABC) was investigated in high and low responder mice, whose ability to respond to (T,G)-A--L is under control of an H-2-linked immune response gene, Ir-1. Unimmunized high and low responder mice have about the same number of ABC in spleen and lymph nodes (6-12 ABC/10(4)). However, after immunization with (T,G)-A--L in aqueous solution, ABC in high responders increase to a much greater extent than they do in low responders. By inhibition of ABC with class-specific anti-Ig sera, it was demonstrated that in nonimmune and primed mice antigen is bound to IgM receptors, which is in agreement with the exclusive production of 19S anti-(T,G)-A--L antibody in primed animals. In contrast, after secondary challenge with antigen, ABC in high and low responder mice have mainly IgG receptors, although under the conditions used for immunization, low responders are not able to produce detectable amounts of 7S anti-(T,G)-A--L antibody. From these results and from the evidence that low responders very probably have a T cell defect, it is suggested that the switchover from IgM to IgG precursor cells can be induced by antigen itself, without the action of specific T cells. Furthermore, the failure of marked proliferation of ABC in low responders after antigenic stimulation is explained by the lack of stimulation by specific T cells. By independent methods it has been shown that all ABC detected in this study are B cells. Preliminary experiments indicate that purified peripheral T cells bind antigen, but much less per cell than do B cells.