A clinical trial was made to evaluate the effectiveness of thymostimulin (TST), a bovine thymic extract, in preventing infections in breast cancer patients treated with polychemotherapy. Fifty-one women treated with adjuvant CMF were randomly divided into two groups: 25 patients received TST plus chemotherapy and 26 patients were subjected to chemotherapy only. Periodic clinical and laboratory checks were performed for at least three months to monitor the occurrence of infections and variations in immunological parameters. Infections (mostly cystitis, conjunctivitis and stomatomucositis) were observed in 37% of the TST treated patients and in 77% of the controls. (p less than 0.005). Serum immunoglobulin concentrations in the two groups were not significantly different, while quantitative analysis of peripheral T lymphocyte subsets showed a higher incidence of decreased T4/T8 ratio in the control group (p = 0.055). Finally, there was a significantly lower incidence of myelotoxicity in the TST treated patients (p less than 0.0005). In conclusion, thymostimulin seems effective in preventing some of the commonest side effects of cancer chemotherapy.