The effect of beta-carotene on tumor immunity was examined with the use of a syngeneic murine tumor system. Oral administration of beta-carotene (120 micrograms/mouse/day) for 9 days from day 1 to the BALB/c mice inoculated sc with 10(7) syngeneic BALB/c Meth A fibrosarcoma cells (Meth A) led to a remarkable rejection against rechallenged Meth A implanted sc on day 10. The growth of Meth 1 fibrosarcoma (Meth 1), another syngeneic tumor of BALB/c origin, as a rechallenge tumor was unaffected by treatment with beta-carotene, thereby suggesting that beta-carotene may augment tumor rejection specific to tumor-specific antigens. Winn assay revealed that the suppressive effect on tumor growth of immune lymph node cells obtained from Meth A-inoculated beta-carotene-treated mice on day 12 was enhanced dose dependently. Primary effector cells responsible for the augmented rejection are Thy-1-positive, Lyt-1-negative, and Lyt-2-positive lymphocytes, presumably cytotoxic T-lymphocytes.