The site-specific glycosylation patterns of two H-2K alleles, k and b, were determined on splenic T cells metabolically labeled with [3H]mannose. Cells from B10, B10.A, (B10 X B10.A)F1, and C3H mice were examined, along with the effect of short- (8 hr) and long-term (36 hr) mitogenic stimulation. For both glycosylation sites (Asn86 and Asn176) of both antigens, 80% of the structures consisted of mono- and bisialylated biantennary N-linked complex oligosaccharides, with the remaining consisting of smaller (probably high mannose) structures. Asn176 of both H-2Kk and H-2Kb contained the same ratio (2.8 to 1) of bi- to monosialylated chains. However, Asn86 of H-2Kb contained a higher ratio (5 to 1), while Asn86 of H-2Kk a lower ratio (1.5 to 1). This difference was seen on antigens isolated from cells of the parental strains as well as from the F1 cross. The glycosylation of H-2Kk did not vary between B10.A and C3H mice. Mitogenic stimulation increased markedly both total [3H]mannose incorporation and the spectrum of N-linked oligosaccharides labeled. For H-2Kk, it had no effect on sialylation, but resulted in a slight under galactosylation of the monosialylated structures at both sites. A comparison of the patterns seen here, determined on nontransformed T cells, with those previously determined on H-2Kk from a B lymphoma line, revealed marked differences in sialylation and branching patterns at both sites. These data indicate that glycosylation differences may be found between highly homologous (91%) alleles of an H-2 gene, even when co-dominantly expressed by F1 cells; however, the patterns do change with mitogenic stimulation, and between normal and transformed cells.