The authors studied 68 patients suffering from breast cancer, with or without lymph node metastasis, who underwent surgery and antitumour therapy (CMF). Twenty-three patients were treated using CMF and 1.5 mg/kg of thymostimulin, 24 with CMF and 1 mg/kg of thymostimulin and lastly, 21 subjects received anti-tumour therapy with CMF alone. Thymostimulin was administered every day for a week prior to surgery; subsequently, it was administered on alternate days for a week and then twice a week for 3 months. The blastogenesis of immunocompetent cells was evaluated. During thymostimulin treatment a higher rate of 3HTdR captation (p < 0.005) by cells stimulated with ConA + IL-2 was observed; these levels tended to increase after 3 weeks and reached statistically significant levels after 3 months of treatment; no significant changes were observed in those patients treated with CMF alone. In addition, the cytotoxic activity of monocytes and NK cells against K-562 cells and against short-lasting cell lines derived from breast carcinoma was also studied. It was observed that this activity increased significantly (p < 0.002) following thymostimulin treatment; this increase was greater in subjects treated with 1.5 mg/kg compared to those treated with 1 mg/kg, but the difference was not statistically significant. The study also evaluated the presence of IL-2 receptors (Tac): thymostimulin treatment for 3 months led to the appearance of receptors, although in restricted numbers, on non-stimulated cells. After IL-2 stimulation, the percentage of cells with Tac receptors increased significantly (p < 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)