We attempted to isolate acquired immune deficiency syndrome (AIDS) virus from a Japanese hemophiliac with AIDS-related complex (ARC). After cocultivation of leukocytes from his peripheral blood with those of a healthy adult, reverse transcriptase activity and AIDS viral antigens were detected by immunofluorescence and radio-immunoprecipitation methods, respectively. Moreover, an electron microscopic study revealed the presence of viral particles consistent with AIDS virus. These retroviruses were further propagated. We designated the first Japanese isolate of AIDS virus as YU-1.