The effect of ascorbic acid on the growth of human mammary tumor xenografts was investigated using the 6-day subrenal capsule assay method. The results showed that ascorbic acid (1 or 5 g/liter) administered in the drinking water significantly inhibited the growth of tumor fragments implanted beneath the renal capsule of immunocompetent mice. The results agree with other work carried out in animal experiments with animal tumors. Administration of ascorbic acid in the mouse diet did not affect the growth of the human mammary tumor fragments within the 6-day experimental period. Tumor growth was inhibited when mice were fed a diet containing ascorbic acid (50g/kg diet) together with cupric sulfate (18 or 90 mg/liter) in the drinking water. The results support the hypothesis that certain oxidation and degradation products of ascorbic acid are active antineoplastic agents for the human mammary carcinoma studied.