The renin activity of plasma from a patient with Wilms' tumor increased markedly after exposure to pH 3.3. When measured by gel filtration, the molecular weight of the renin present both in plasma and tumor extract of this patient was 60,000; in contrast, the molecular weight of normal plasma and kidney renin did not exceed 43,000. Acid activation did not significantly alter the molecular weight of the big renin from Wilms' tumor. Antibodies prepared against normal human renin showed cross-reactivity with big renin. Both normal and big renin showed identical changes in activity over pH range from 4 to 8. Thus, the big renin secreted by the Wilms' tumor may be a prohormone form of normal plasma renin.