The enhanced effect of CDDP combined with caffeine against P-388 leukemia was investigated. No synergistic effect was shown after one hour simultaneous treatment with CDDP and caffeine. But the growth inhibition was enhanced by the addition of caffeine after one hour treatment with CDDP. It is suggested that caffeine inhibits DNA repair by the reduction of CDDP-induced elongation of G2 + M phase. The increase in life span of mice after ip transplantation was observed by frequent ip injections of caffeine after CDDP injection. It is suggested that the effect of intraperitoneal administration of CDDP against peritoneal metastasis is enhanced by combination with caffeine.