To determine which cells in kidney grafts are infected with human cytomegalovirus (HCMV) before and after transplantation, kidney specimens were studied by in situ hybridization with 35S-labeled DNA probes representing HCMV immediate-early and late genes. Pretransplantation biopsies and serial posttransplantation biopsies were obtained from 7 renal grafts. All of the transplant recipients were HCMV-seronegative at the time of transplantation and all developed primary HCMV infections. HCMV nucleic acids were not detected in biopsies taken from the healthy donor kidneys before transplantation. However, biopsies taken at various intervals after transplantation showed abundant hybridization with HCMV immediate-early and late gene probes. Virtually all of the hybridizing cells were mononuclear inflammatory cells in the interstitial spaces of the kidney. Occasional hybridization was seen with renal tubular or glomerular cells. No cytomegalic cells were seen. Biopsy specimens taken after systemic anti-HCMV chemotherapy with phosphonoformate showed no uniform reduction in HCMV gene expression. These studies demonstrate that the principal HCMV-infected cells in kidneys of renal transplant patients with primary HCMV infections are infiltrating inflammatory cells.