Forphenicinol (FPL) is a low molecular immunomodifier derived from forphenicine, a microbial product found by Umezawa and co-workers. We studied the antitumor effect of FPL, cyclophosphamide (CY), and the combination of the two on several syngeneic murine tumors. The tumors used were mammary carcinoma, L1210 leukemia, B16 melanoma, Lewis lung carcinoma, and glioblastoma. A single ip injection of CY on Day 1 followed by eight consecutive daily oral doses of FPL beginning 6 days after tumor inoculation showed strong cooperation in curing syngeneic mammary carcinoma inoculated intradermally in C3H/HeN mice, most mice being cured of the tumor by the combination therapy and subsequently having acquired strong specific immunity. Treatment with FPL alone (either pre- or post-treatment) also significantly inhibited the growth of the mammary tumor. FPL and CY also showed cooperation in inhibiting the growth of L1210 leukemia transplanted intradermally into CDF1 (BALB/c X DBA/2) mice and markedly prolonged the survival time but FPL treatment alone had no effect. The FPL-CY treatment also affected Lewis lung carcinoma and glioblastoma in syngeneic C57BL/6 mice and produced therapeutic synergism. FPL alone significantly inhibited the growth of B16 melanoma in C57BL/6 mice as well as the syngeneic mammary carcinoma in C3H/HeN mice. These findings suggest that oral administration of FPL in combination with chemotherapeutic agents can be used for treating cancer without causing toxicity, because of the synergistic efficacy of the combination.