Two distinct T-cell glycoforms of CD43 result from differential glycosylation of a single gene product in vivo. The 115 kDa glycoform carries mainly tetrasaccharides and is a pan T-cell marker, whereas the 130 kDa glycoform carries mainly hexasaccharides and is associated with T-cell activation. CD43 has been shown to play a role both in enhancing and inhibiting cell adhesion; however, the function of the individual glycoforms is unknown. We have examined the distribution and regulation of the CD43 glycoforms in a murine model of acute graft-versus-host disease (GVHD) using monoclonal antibodies (mAbs) S7 and 1B11 specific for the 115 and 130 kDa CD43 glycoforms, respectively. An increase in T-lymphocyte CD43 130 kDa expression occurred during GVHD from day 4 onwards and coincided with splenomegaly and upregulation of the beta 1-6GlcNAc transferase (C2GnT), the key enzyme responsible for the addition of complex O-glycan branching to CD43. When T-lymphocyte subsets were examined for CD43 expression, we found that in GVHD, both CD43 glycoforms were upregulated on CD4+ T cells. However, in CD8+ T cells, CD43 115 kDa was downregulated while CD43 130 kDa was dramatically upregulated, such that two distinct CD8+1B11+ T-cell subsets were observed. These data demonstrate differential expression of the CD43 glycoforms in both resting and activated CD4+ and CD8+ T cells, and suggest that glycosylation differences between the CD43 glycoforms may reflect participation in the different functions of these T-cell subsets in immune disorders in vivo.