The effect of treatment with the double-stranded polynucleotide complex polydenylate with polyuridylate [poly(A) with poly(U)] on tumor development in C3H/He mice was evaluated. Poly(A) with poly(U) was injected in newborn females, and mice were observed for 380 days. During this experimental period 42 percent of treated mice developed tumors, while the incidence in the control group was 63 percent. This difference was statistically significant.