B6 alloantigens in supernates from one-way mixed lymphocyte reaction (MLR) cultures of AKR T cells against B6 lymph node cells rebound specifically to (B6)AKR-T-cell blasts after overnight incubation and recovery of these blasts from trypsin treatment. A similar specificity was observed with the binding of SJL alloantigens to (SJL)AKR-T-cell blasts. In both cases, the corresponding alloantigens were rebound several-fold more efficiently than control alloantigens. In a different assay system, T-cell receptors were studied with anti-idiotypic sera. These antisera were raised by repeated injection of purified (B6)AKR-T-cell blasts into (AKR X B6)F1 hybrid mice. These F1a(AKRaB6) sera reacted with the majority of (B6)AKR-T and (B6)(AKR X SJL)F1-T-cell blasts. They also reacted with a lower, though sizable, number of (SJL)AKR-T- and (SJL)(AKR X B6)F1-T-cell blasts. No reaction was observed with (B6)SJL-T, concanavalin A-activated AKR T-cell blasts, normal B6, (AKR X B6)F1, and (AKR X SJL)F1 T cells. Normal AKR T cells were positive only minimally above background. F1a(AKRaB6) sera could be made specific for (B6)AKR-T and (B6)(AKR X SJL)F1-T-cell blasts by absorption of contaminating antibodies with (SJL)AKR-T-cell blasts. Finally, it was shown by competition experiments that the receptors on MLR-activated T-cell blasts that bind alloantigens were the same as those binding anti-idiotypic antibodies. In addition, it was found that at least a fraction of alloantibodies share common idiotypic determinants with receptors on MLR-activated T-cell blasts.