Random additional treatment with Thymopentine was administered to a group of 46 mastectomised women in order to assess its possible anti-leucopenic efficacy in relation to cytotoxicity caused by anti-blastic drugs. Results reveal a certain degree of efficacy in reducing the incidence of leucopenia when treatment was administered continuously from the start to the end of chemotherapy. This aspect plays an extremely important role in avoiding the protraction of the normal interval between one cycle of anti-blastic chemotherapy and the next.