Biosynthesis of core2 O-glycans on T cell surface glycoproteins is essential for their interactions with selectins expressed by activated endothelium, and may also regulate susceptibility to apoptosis. Beta-galactoside alpha2,3-sialyltransferase-I (ST3Gal-I) is a major inhibitor of core2 O-glycan formation on CD43 and CD45 in naive T cells. Here we show that ST3Gal-I mRNA is extensively expressed during Th2, but not Th1 differentiation. Consistent with this, developing Th1 cells display the non-sialylated core1 galactose (Gal1-3GalNAc-Ser/Thr) and strongly react with peanut agglutinin, while Th2 cells do not. These Th subset differences are also associated with greater expression of the high molecular glycoform of CD43 on the surface of Th1 cells compared with Th2 cells, and similar differences in surface expression of sialyl Lewis-X. We suggest that lack of ST3Gal-I expression in Th1 cells allows the formation of surface core2 O-glycans and supports their interactions with endothelial selectins.