[(125)I](T,G)-A--L-binding T cells have been studied in mice whose ability to mount an immune response to (T,G)-A--L is under control of the H-2-linked Ir-1A gene. Nonimmunized high and low responder mice have approximately the same frequency of T-ABC. Following immunization, T-ABC proliferated only in high responders, but not in low responders, indicating expression of Ir-1A in T cells. When, for comparison, [(125)I]arsanyl-mouse serum albumin binding B and T cells were investigated in mice whose antibody response to the hapten arsanyl is controlled by an allotype-linked Ir gene, it was found that following immunization the number of B-ABC increased only in high responders. In contrast, T-ABC proliferated to the same extent in both high and low responders, suggesting exclusive expression of the allotype-linked Ir gene in the B-cell line. Preliminary studies indicate that anti-Ia sera inhibit neither B-ABC nor T-ABC.