One hundred kidney allograft biopsies from 85 patients and tissue specimens from 31 failed allografts were analysed by immunohistochemistry (IMH) with a monoclonal antibody against a cytomegalovirus (CMV) antigen and by in situ hybridisation (ISH) with a biotinylated CMV DNA probe. Six clinically and serologically CMV-infected patients with AIDS served as positive controls. Biopsies from six seronegative donor kidneys and autopsy specimens from five clinically and serologically negative patients served as negative controls. Adequate serology in 56 patients showed 50% to be actively CMV infected. No statistically significant difference was found between the frequency of acute rejection in the seropositive and the seronegative patients. Four seropositive transplanted patients contained CMV DNA or CMV antigen in the biopsies and in the grafts. Both methods gave a negative result in 15 patients with glomerulopathy. Expression of MHC class I and II antigens in 17 of the biopsies demonstrated no relation to positive serology or local demonstration of CMV. In conclusion, (1) CMV was not renotropic, (2) there was no correlation between (a) CMV and acute rejection, (b) CMV and glomerulopathy, or (c) CMV and MHC class I and II antigen expression. The study suggests that CMV does not play a major role in rejection.