Five month-old female rats were given a mixture of Sr-90 and Cs-137 in drinking water in the dose of 0.1 and 0.2 microCi/day per animal over 12 months. Some animals received 12 monthly courses of a synthetic immunomodulating dipeptide--thymogen in the dose of 5 micrograms/animal for 5 consecutive days. Radionuclide-treated rats showed higher occurrence of tumors on the whole and of breast adenocarcinoma, in particular. Thymogen was shown to inhibit Sr-90- and Cs-137-induced radiation carcinogenesis, namely, a decrease in the total tumor and cancer occurrence was observed. The animals receiving thymogen alone showed longer life span, slower rate of aging and lower overall tumor and cancer occurrence. In this study, the ability of a synthetic peptide immunomodulator--thymogen to inhibit spontaneous and radionuclide-induced carcinogenesis in female rats was first established.