The synthetic double-stranded RNA, polyinosinic-cytidylic acid, inhibits the growth of some tumors in mice. Two days after implantation of a reticulum cell sarcoma, a lymphatic lymphoma, a fibrosarcoma, two leukemias, and a human adenovirus 12-induced tumor, treatment of groups of mice resulted in decreased growth rates of the tumors and increased survival times of the animals. In the two tumors tested (the reticulum cell sarcoma and the adenovirus 12-induced tumor) initiation of treatment after the tumor was grown to moderate size caused a regression of the tumor. In the case of the reticulum cell sarcoma, the tumor had not reappeared in some of the animals two months after cessation of treatment.