Clinically, there is an association between CMV infections and the occurrence of rejection after renal transplantation. Adhesion molecules such as intercellular adhesion molecules (ICAM) and MHC Ags are thought to be important in the induction and amplification of the rejection process. Therefore, we studied ICAM-1 and MHC expression after CMV infection or stimulation with cytokines. Cultured proximal tubular epithelial cells (PTEC) were stimulated with cytokines or infected with CMV. MHC class I, class II, and ICAM-1 expression were determined by radioimmunoassay. IFN-gamma induced class II and ICAM-1 expression. Small concentrations of IFN-alpha inhibited the IFN-gamma induced class II expression. CMV-infected PTEC displayed increased levels of ICAM-1 and class I expression. This enhancement is a direct effect of the virus on the infected cells and not mediated by soluble factors. Although MHC class II expression is not directly enhanced by CMV, infected PTEC display a normal increase of class II expression after stimulation with IFN-gamma.