A comparative study of the antitumor effect of murine recombinant interferon(beta) less than Mu-rIFN(beta) greater than and murine recombinant interferon(gamma) less than Mu-rIFN(gamma) greater on B16-F10 melanoma was conducted. Administration of Mu-rIFN(gamma) i.p. into C57BL/6 mice on days 1 to 7 produced a higher suppressive effect than Mu-rIFN(beta) both on the growth of s.c. implanted tumor and on the formation of artificial pulmonary metastasis. Pharmacokinetic study of Mu-rIFN(gamma) demonstrated that high plasma levels were retained for a long time. In clonogenic assay, Mu-rIFN(gamma) at 1000 units/ml showed about 80% inhibition of colonies of B16-F10 melanoma. However, Mu-rIFN(beta) hardly inhibited the colonies, even at 1000 units/ml. Augmentation of natural killer (NK) cytotoxicity was much greater with Mu-rIFN(beta) than Mu-rIFN(gamma), whereas Mu-rIFN(gamma) enhanced the cytotoxicity of peritoneal macrophages more strongly than Mu-rIFN(beta). Injection of Mu-rIFN(gamma) i.p. 1 day before tumor challenge also inhibited the formation of pulmonary metastasis of B16-F10 melanoma. However, pretreatment of mice with carrageenan significantly suppressed the inhibitory effect of Mu-rIFN(gamma). From these results, it is suggested that the inhibitory effect of Mu-rIFN(gamma) on the tumor growth and metastases of B16-F10 melanoma is mediated partly by direct antitumor effect and partly by the activation of macrophages, and that the augmentation of NK activity contributes mainly to the antitumor effect of Mu-rIFN(beta).