Human T-cell leukemia virus type I was induced by N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) and 5-iodo-2'-deoxyuridine (ldUrd) in the MT-1 cell line. Virus expression was monitored by immunofluorescence microscopy with GIN-14, mouse monoclonal antibodies directed toward Mr 19,000 and Mr 28,000 protein-specific virus polypeptides. MNNG (0.1 micrograms/ml) and ldUrd (50 micrograms/ml) both induced virus synthesis in MT-1 cells. MNNG-induced virus expression peaked between 24 and 48 h of incubation, whereas ldUrd induced maximum virus expression between 48 and 72 h of incubation. Superinduction resulted when MNNG was added to cells induced 48 h previously with ldUrd, but not with concomitant treatment. 13-cis-Retinoic acid, retinol, retinol aldehyde, and retinol acetate (10(-6) to 10(-9)M) were concomitantly added with ldUrd to MT-1 cells for 24, 48, and 72 h incubation. All inhibited virus induction to various degrees. The retinoids were ranked as to inhibitory activity: retinol greater than retinoic acid greater than retinol aldehyde greater than retinol acetate. The most sensitive period for inhibiting ldUrd induction by retinoic acid was 24 h postinduction or with concomitant treatment. Vitamin C and vitamin E inhibited ldUrd induction most effectively with 48 h incubation. Retinol and vitamin C also inhibited virus induction by MNNG. None of the retinoids, vitamin C, or vitamin E significantly inhibited virus expression in noninduced cells or were toxic to the cells at the concentrations used in these experiments.