Adjuvant treatment with polyadenylic-polyuridylic acid poly A-poly U was tested in 300 patients with operable breast cancer who had all received the same locoregional treatment. They were randomized into two groups; 155 patients receiving 30 mg poly A-poly U i.v. once a week for 6 weeks and 145 controls. Overall survival was significantly improved in the poly A-Poly U group (P less than 0.05). The most striking difference (less than 0.03) was observed in the group of positive node patients, who had a 5-year relapse-free actuarial survival rate of 71% versus 47% in the controls.