An established model for latency by mouse cytomegalovirus (MCMV) was used to assess the effects of leukaemia on reactivation of MCMV. After inoculation of 400 spleen focus-forming units of Friend leukaemia virus (FLV), a significant number of mice yielded MCMV in salivary glands and kidneys 4 to 5 weeks later. Administration of cyclophosphamide during FLV infection increased the percentage of MCMV-positive animals. These results suggest that cytomegalovirus infections in leukaemic patients may reactivate as a result of the leukaemia itself and may be exacerbated by chemotherapy.