Antineoplaston A10, a human urinary product of minimal toxicity and demonstrable antineoplastic activity, was examined as a modulator of spontaneous mammary tumour development in C3H+ mice. These animals normally express the viral response with appearance of tumours in greater than 95% of the mice by 9-10 months of age. Inclusion of A10 as a 1% dietary supplement from the age of 3 months dramatically increased the disease-free interval. At 10-11 months of age, none of the animals had developed tumours, and the incidence reached 95% only at 21 months of age. In spite of this effect, the survival interval following tumour detection could not be prolonged by treatment with A10. Male C3H+ mice, normally fully protected against spontaneous mammary tumour by their testicular androgens, can be made susceptible to such tumorigenesis by castration. The age at which castration is performed influences the age of onset of the disease: castration at one month leads to initial appearance at 5 months of age; castration at 2 months to appearance at 7-8 months. When such animals are maintained on A10-containing diets following castration, the onset of tumour appearance is delayed until 7 months and 10 months for the 1- and 2-month castrated animals respectively. The results indicate a clear inhibitory action of A10 on MMTV-induced spontaneous mammary tumour occurrence in a manner similar to the protective effect of androgens.