The BDF1 mice when injected with 106 L-1210 ascites leukemia cells die in 8 days. When treated from days 1 to 7 with 5 mg/kg of polyinosinic acid : polycytidylic acid (1:1) they have a 25-40% increase in life span. The material is effective when given i.p. or s.c., but not by oral route, and the component polyribonucleotides are also inactive. The activity of the double-stranded complex in prolonging survival is not due to in vitro cytotoxic or cytostatic activity. It is proposed that the activity of poly I:poly C is due to the induction in the host on interferon which then slows the multiplication of the L-1210 cells in vivo.