We have previously shown that noncytotoxic doses of camptothecin (CPT), a topoisomerase I-specific antagonist, inhibit retrovirus replication in acutely and chronically infected cells. To evaluate the efficacy of CPT as an antiretroviral drug in vivo, we injected newborn BALB/c mice with Moloney murine leukemia virus and adult NFS mice with Friend spleen focus-forming virus. The Moloney murine leukemia virus-injected mice developed lymphoma, and the Friend spleen focus-forming virus-injected mice developed erythroleukemia. CPT, administrated together with the virus or 1 or 2 days after virus injection, prevented the onset of the disease in both cases. We showed that repeated CPT treatments increased the effectiveness of the drug when administrated 3 days after virus injection. This ability of CPT to inhibit retrovirus-induced disease in vivo without causing any apparent toxic side effects suggests its application as a legitimate remedy for the treatment of retroviral diseases.