Cytotoxic T lymphocytes (CTL) activated in H-2 identical, Qa-1 disparate mixed leukocyte cultures recognize H-2-nonrestricted target antigens indistinguishable by strain or tissue distribution from serologically defined Qa-1 antigens. Cloned Qa-1-specific CTL define determinants encoded by four Qa-1 genotypes; we used anti-Qa-1 sera in antibody blocking experiments to determine if these determinants reside on molecules recognized by Qa-1-specific antibodies. Antisera containing Qa-1.1-specific and TL-specific antibodies blocked recognition of two CTL-defined determinants associated with Qa-1a. Although both Qa-1 and TL molecules are expressed on activated T cells from appropriate strains, our studies indicated that the CTL recognized Qa-1, not TL. In addition, anti-Qa-1.2 serum inhibited CTL recognition of Qa-1b- and Qa-1c-encoded determinants. Qa-1d target cells are unique in that they express determinants recognized by anti-Qa-1a CTL and by anti-Qa-1b CTL. Killing of Qa-1d targets by anti-Qa-1a CTL was not inhibited by anti-Qa-1.1 serum, but was partially inhibited by anti-Qa-1.2 serum. Cytotoxicity of Qa-1d cells by one anti-Qa-1b CTL clone was inhibited by both anti-Qa-1.2 and anti-Qa-1.1 sera, indicating close association of both serological determinants with the determinants recognized by the CTL. Thus, all of the CTL-defined Qa-1 determinants resided on molecules recognized by Qa-1-specific antibodies, but anti-Qa-1a CTL and Qa-1.1-specific antibodies did not have identical specificities.