Qa-1-specific, H-2-unrestricted cytotoxic T lymphocytes (CTLs) generated from reciprocally immunized B10.BR (H-2k, Qa-1a) and CBA (H-2k, Qa-1b) mice, and immunoprecipitation of cell surface Qa-1 were used to examine Qa-1 region determinant expression by H-2r and H-2f mice. The H-2b, Qa/Tla congenic mice B6-Tlaa and C57BL/6J (B6) were used as prototype Qa-la and Qa-1b strains, respectively. Cells from H-2r strains expressed determinants recognized by B10.BR anti-CBA CTLs. Reciprocal cold target inhibition of cytolysis demonstrated that some, but not all, Qa-1b-associated determinants recognized on B6 were expressed by H-2r, Qa-1c cells. In contrast, cells from H-2f strains expressed Qa-1 determinants recognized by both B10.BR anti-CBA and by CBA anti-B10.BR CTLs. Cold target inhibitions indicated that the H-2f, Qa-1d cells expressed some, but not all, Qa-1a-associated antigens recognized on B6-Tla and did not express the same Qa-1b-associated antigens as B6. Furthermore, H-2f strains expressed cell surface antigens immunoprecipitable by antisera specific for both Qa-la and Qa-1b-encoded determinants; these molecules were of the same molecular weight as those immunopre- cipitated from B6 and B6-Tlaa. These data suggest that CTLs and antisera define the same, closely related Qa-1 determinants encoded by a polymorphic locus. © 1982 by The Williams and Wilkins Co.