Our previous studies demonstrated that CTL responses to the newly identified minor H-43a and H-43b alloantigens induced in H-43b and H-43a responder mice, respectively, utilized the same H-2Kb (Kb) MHC class I restriction element, whereas many allelic class I products were not utilized. Also, a single i.v. injection of H-43b recipient mice with spleen cells (SC) from H-43 congenic mice induced specific and lasting CTL tolerance to H-43a rather than priming. We have produced a H-43b C3H.AU mouse strain (H-2p), which we regard as H-43 congenic to the C3H.NB (H-2p, H-43a) strain. Using this strain we demonstrate that Kp and/or Dp alleles can serve as class 1 restricting elements in the CTL responses to H-43a and H-43b alloantigens. Injection of H-43a recipient mice with antigenic H-43b SC from the paired H-43 congenic abrogated the anti-H-43b CTL response, irrespective of MHC class I specificity, i.e., in the contexts of Kb and of Kp and/or Dp elements. A corresponding result was obtained upon injection of H-43b recipient mice with congenic H-43a SC. The tolerance-inducing capacity of H-43a congenic SC was so strong that injection of these cells into H-43b responders that had previously received immunogenic H-43a SC completely blocked the previously activated Kb-restricted memory CTL activity specific for the H-43a alloantigen.