The experimental infection of six cynomolgus monkeys with adult T-cell leukemia virus (ATLV) was attempted. Three animals were inoculated with living MT-2 cells and three with cell-free ATLV. All animals developed an antibody response to virus-specific glycopolypeptides and viral core polypeptides. ATLV-specific antigens appeared in peripheral lymphocytes from all six animals. Virus expression persisted in all animals. Up to 40 weeks after inoculation no animal developed any symptom of leukemia.