Corynebacterium parvum has been used as an anti-tumor agent in both animal experiments and clinical trials; nevertheless it is not well established the mechanism of action, since both T-dependent and T-independent ways of action have been proposed. On the other hand, mononuclear phagocytic system activation cannot be ruled out in the immunomodulation carried out by C. parvum. In the attempt to better understand the effects of C. parvum on macrophage functions, experiments were carried out on tumor-bearing rats with a decline of peritoneal macrophage activities as a result of the tumor development. For this purpose three different preparations of C. parvum were investigated: a) the first preparation was obtained by heat-inactivation of bacteria at 60 degrees C for 1h; b) the second by formalin-treatment of bacteria; c) the third obtained from Burrough Wellcome as a formaline-killed suspension in 0.01% w/v thiomersal saline. Each preparation was used at 7 mg/ml dry weight and administered subcutaneously, three days after tumor grafting. The results showed that all three preparations were capable to hinder the tumor mass growth. Macrophage functions as phagocytosis, intracellular killing, intrinsic and extrinsic anti-Herpes Simplex Virus activities, were restored to normal values by each of the preparations used. Nevertheless, concerning the chemotactic activity a discrepancy was noted among the three preparations, since the heat-inactivated one was able to carried out a complete restoration.