Mice expressing mutant H-2K b alleles were tested for their ability to generate cyt0toxic effector T-cells specific for the non-H-2 histocompatibility alloantigen H-4.2. Cytotoxic effectors specific for H-4.2 are preferentially restricted by the K b allele. Mutant K b alleles were observed to differentially regulate the magnitude of the H-4.2-specffic cytotoxic effector response. Mice expressing the K binS, K bin6, K bin7, and K bin9 alleles generated cytotoxic T-cells to the same level as mice expressing the wild-type K b allele. K bin8 and K b"l~ responders generated intermediate levels of effectors, whereas K bin1, K bin3, and K b'~Β° responders did not generate detectable levels of cytotoxic effectors. K bin4 responders produced high levels of H-4.2-specific cytotoxic effectors that were variably reactive with wild-type K b antigens with no H-4.2. The ability to generate H-4.2-specific effectors generally correlated with (1) the ability of mutant K b molecules to present H-4.2 to wild-type Kb-restricted effectors, and (2) the position of the respective amino acid interchanges on the K b molecule. Mutations that altered the amino acid sequence in the vicinity of the disulfide bond in the C1 domain had the greatest deleterious effects on Kb-controlled responsiveness to H-4.2. The only exception was the K b"l~ intermediate responder, which differs from K b"3 in both responsiveness and in a single amino acid interchange. Therefore, the amino acid sequence in the vicinity of the disulfide bond in the C1 domain plays a prominent role in determining the H-4.2specific immune response potential. These observations are the first to clearly demonstrate association between particular MHC gene product, amino acid sequences and immune responsiveness.