Cytotoxic T lymphocytes generated by sensitization of B10.Br (H-2(k), Qa-1a) cells in vivo and in vitro with CBA or AKR (H-2(k), Qa-1b) spleen cells were used to evaluate fresh determinants recognized on fresh and mitogen-stimulated T and B cell targets. Using targets from the congenic mouse strains C57BL/6J (Qa-1b) and B6-Tlaa (Qa-1a) to demonstrate Qa-1 specificity, we found that mitogen-stimulated T and B cell blasts both expressed Qa-1 determinants. Reciprocal cold target inhibition experiments revealed that the same antigens were recognized on Con A and anti-Thy-1.2 plus C-treated LPS blasts. Furthermore, splenic and thymic Con A blasts expressed indistinguishable Qa-1-associated antigens. In a similar experiment, fresh lymphocyte targets were used to demonstrate that unstimulated splenocytes and thymocytes expressed the same antigens. Anti-Thy-1.2 plus C-treated freash splenic B cell targets also expressed Qa-1 antigens, none of which were apparently unique, as demonstrated by inhibition of cytotoxicity by immunoglobulin-negative splenic T cells. In addition, using CBA anti-B10.BR effectors, cytotoxicity of anti-Thy-1.2 plus C-treated fresh B6-Tlaa splenic B cells was inhibited by anti-Qa-1 serum. Thus, Qa-1 determinants were expressed on resting and activated T and B lymphocyte subpopulations, and no qualitative differences were identified betweeen determinants recognized by Qa-1-specific cytotoxic effectors on the various lymphoid subpopulations.