Resistance and susceptibility to Marek's disease (MD) are strongly influenced by the chicken major histocompatibility complex (MHC). In this study, splenic lymphocytes from MD-resistant and MD-susceptible chickens of three MHC genotypes (B21/B21, B19/B21, and B19/B19) were analyzed by flow cytometry for MHC surface expression. In the spleen, constitutive MHC class I surface expression was found to be highest in homozygous B19, lowest in homozygous B21, and intermediate in heterozygous B19/B21 animals. This was observed on CD4(+), CD8(+), and Bu-1(+) splenic lymphocytes. Chickens of all three genotypes were subjected to infection with MD virus (GA strain) and spleen samples from infected as well as MHC-matched negative controls were analyzed at 1, 4, and 8 wk post-infection (p.i.). It was observed that MDV induced an increase in MHC class I expression late in the infection. Thus, MHC class I was increased on the surface of CD4(+) cells from infected chickens of all genotypes at 4 and 8 wk p.i. compared with negative controls. Also, MHC class I expression was increased on CD8(+) cells from infected chickens of all genotypes at 4 and 8 wk p.i., except for the homozygous B19 animals, that showed no increase at 8 wk p.i. MDV-induced differences in MHC class II surface levels were also found. Thus, MHC class II expression was increased on CD4(+) cells from infected B19/21 and B21/B21 chickens at 4 wk p.i., and also at 8 wk p.i. on CD4(+) cells from infected B19/B21 animals. MHC class II expression was increased on CD8(+) cells from infected chickens of all genotypes at 4 wk p.i. These findings suggest that MDV infection in vivo increases the level of MHC surface expression on splenic T cells, indicating a possible role in immunity against MDV.