To analyze the expression of different human cytomegalovirus (HCMV) antigens in gastric biopsy samples from a renal transplant recipient suffering from HCMV gastritis, monoclonal antibodies (MAbs) were used in immunohistochemical analyses. In samples obtained before the start of specific therapy with ganciclovir, MAbs against immediate-early (E13), early (CCH2), and late antigen (XP1) reacted with cells in the smooth muscle, stromal, and epithelial layers. MAb E13 stained morphologically altered and unaltered cells; MAbs CCH2 and XP1 predominantly stained cytomegalic cells. After therapy with ganciclovir was begun, the three viral antigens were almost exclusively found in the epithelial cell layer, whereas viral antigen appeared to be cleared from smooth muscle and stromal cells. All cells stained by MAb E13 in these samples showed characteristic morphologic alterations, in contrast to cells in earlier samples. The changes in the pattern of viral antigen expression may reflect the effects of antiviral therapy.