The antitumor effects of vitamin A and its related substances, vitamin E, vitamin K, beta-carotene, ubiquinone, phytol, and squalene, were examined using a syngeneic murine tumor system. Intraperitoneal administration of these substances (0.19 mumol/mouse/day) slightly suppressed the growth of Meth A fibrosarcoma cells inoculated s.c. into Balb/c mice. Administration of all test substances except beta-carotene significantly suppressed the growth of Meth A fibrosarcoma cells rechallenged in Meth A-primed mice on day 10, but did not influence the growth of Meth 1 fibrosarcoma cells (another syngeneic tumor of Balb/c origin) rechallenged in Meth A-primed mice on day 10. The growth of Meth A tumor cells was suppressed when Meth A was inoculated together with lymph node cells obtained from the Meth A-primed Balb/c mice treated with vitamin A, vitamin E, phytol, or squalene. Our findings suggest that certain constituents in green-yellow vegetables may contribute to the prevention of cancer by augmenting an immunological response against tumor cells in the early stages of carcinogenesis.