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An otherwise conventional diet supplemented with retinyl acetate (vitamin A acetate; VAA) increased a specific antitumor resistance in vivo in mice, and this appeared to be due to the enhancement of immune functions rather than a direct antitumor activity. A range of VAA doses (up to 0.8 g VAA per kg of diet), fed for more than 6 months, did not induce any obvious signs or symptoms of hypervitaminosis A. The augmentation of resistance to transplanted tumor was linearly dependent on the dose of VAA. There was also a positive linear relationship between the resistance to transplanted tumor and the length of exposure to supplementary VAA relative to tumor inoculation time. The maximum resistance to transplanted tumor was observed in VAA-fed mice when the enrichment of the diet with VAA started three or more weeks before inoculation of tumor, whereas initiation of the VAA diet on the day of tumor administration or later had a negligible effect on the growth of tumor.