The enhancement of antitumor activity of Propionibacterium avidum (P. avidum) in combination with Neurotropin (NSP) was investigated in C3H mice-MH 134 tumor system. P. avidum (0.5 mg) and NSP (20 mg/kg) were administered on day 2 and from day 1 to 7 after tumor inoculation, respectively. When mice were treated with P. avidum in combination with NSP, a significant prolongation in survival days was observed (P less than 0.01). Treatment with P. avidum alone produced prolongation in survival days, but NSP did fail. Increase of Con-A induced suppressor cell activity and depressed proliferative response of spleen cells were observed by the treatment with P. avidum. However, recovery of proliferative response to normal level and disappearance of suppressor cell activity were observed when NSP was combined. Thus, treatment by P. avidum in combination with NSP produced a significant prolongation in survival days and it may be depending on macrophage activation by P. avidum and on the restoration of T cell functions by NSP.