Cytomegalovirus (CMV) is presumed to cause latent infection, but the sites of infection are incompletely known. We propose that latent murine cytomegalovirus is present in kidney and may be reactivated by explanation. Immunosuppressive agents and allogeneic stimuli may enhance this process. Balb/c mice were infected 11 to 14 months previously with 105 pfu of Smith strain murine cytomegalovirus intraperitoneally. Kidneys from 15 infected and nine uninfected mice were washed, minced into 1 to 2 mm2 explants and placed into separate tissue culture wells containing a mouse embryo fibroblast monolayer. Explants were untreated or treated with azathioprine, cyclophosphamide, anti-thymocyte serum, or allogeneic lymphocytes. Daily observations for CPE and passage of supernatant to fresh mouse embryo fibroblast were done. Standard cultures of blood, kidney, and salivary gland were negative. However, virus was isolated from the explants of 8/15 animals, with a reactivation time of 30 to 70 days. No significant difference in reactivation time was noted between treated or untreated explants. Restriction enzyme analysis of viral DNA confirmed identity with the original strain. These data show that latent murine cytomegalovirus is present in murine kidney tissue and may be reactivated by explantation.