The Qa-1b allele is assigned to strains whose peripheral lymphocytes are not lysed by a standard anti-Qa-1 serum. In order to investigate whether Qa-1b represents a single allele or several alleles that have been grouped together as 'not a', the authors studied the activity of cytotoxic T lymphocytes (CTL) specific for Qa-1b-associated antigens. Cold target inhibition experiments were performed to test the ability of various Qa-1b targets to inhibit the lysis of 51Cr-labeled B10 blasts by B10.BR anti-C3H and C57BR anti-AKR CTL. Qa-1a targets did not inhibit lysis relative to controls syngeneic with the CTL. In contrast, Qa-1b cold targets strongly inhibited the lysis of labeled B10 targets. The authors found no differences in inhibition among 16 Qa-1b strains representing six H-2 haplotypes and nine different backgrounds. These data indicate that all of these Qa-1b strains express the Qa-1 antigens shared by B10 and C3H or AKR, and thus suggest that Qa-1 is markedly less polymorphic than H-2. Experiments were also performed to test the possibility that Qa-1 associated antigens are involved in the recognition of minor histocompatibility determinants. CTL were generated in secondary cultures between strains identical at Qa-1 and at H-2, but differing at minor histocompatibility loci. These CTL were tested for their ability to lyse targets bearing the minor histocompatibility antigens of the stimulators, and sharing either Qa region or H-2 antigens with the stimulators. In contrast with the well established role for H-2K and H-2D antigens in the recognition of conventional determinants, the authors found that in both Qa-1a and Qa-1b strain combinations, Qa region homology was neither necessary nor sufficient for minor locus-specific lysis.