A contribution of N-linked oligosaccharides to determinants recognized by alloreactive cytotoxic T lymphocytes has not been demonstrated. Employing cloned CTL and tunicamycin, an inhibitor of protein glycosylation, we found that carbohydrate addition was required for the formation of two of six Qa-1 determinants. The other determinants were detectable on nonglycosylated Qa-1 molecules, similar to observations in most reports that allodeterminants on class I molecules are not dependent on glycosylation for serologic detection. Examination of TM-treated, Con A-activated lymphoblasts revealed a direct correlation between the determinants defined by the reactivity of CTL clones with target cells from four Qa-1 genotypes and their dependence on carbohydrate side chains for expression. Most anti-Qa-1b CTL clones recognized either a glycosylation-dependent determinant found only on Qa-1b Qa-1(c) or glycosylation-independent determinants on both Qa-1b and Qa-1 cells. Similarly, clones that killed only Qa-1a cells recognized a glycosylation-independent determinant. However, clones reactive with both Qa-1a and Qa-1(d) cells recognized a glycosylation-dependent determinant on Qa-1a molecules and a glycosylation-independent determinant on Qa-1(d) molecules. This result indicates that such clones recognize cross-reactive conformational determinants, not carbohydrate itself. Thus, N-linked oligosaccharides serve to stabilize the conformation of some Qa-1 determinants, but others remain intact on nonglycosylated molecules. The absence of similar data for H-2K/D/L molecules suggest that a reexamination of other class I antigens with cloned CTL is in order to determine whether Qa-1 molecules are unique.