The antitumor activity of three different species of Propionibacteria (P. granulosum, P. avidum and P. acnes) against murine Sarcoma 180 and Moloney-Sarcoma-Virus-induced tumor (MSV tumor) was investigated. All three Propionibacteria injected intraperitoneally or intratumorally in multiple doses of 1 mg per mouse appeared to be effective regarding the retardation of growth and the stimulation of regression of Sarcoma 180 in CFW mice. Moreover, the application of Propionibacteria significantly prolonged the survival of Sarcoma-180-bearing mice. No significant differences were found in antitumor effect against Sarcoma 180 between P. granulosum, P. Avidum and P. acnes, but in general, intratumoral injections of all three strains were more effective than intraperitoneal applications. All three strains of Propionibacteria were equally ineffective regarding the retardation of growth and the stimulation of regression of MSV tumors in adult NMRI mice, as assessed after intraperitoneal injections of P. granulosum, P. avidum or P. acnes (1 mg per mouse) 3 days prior to inoculation of virus particles.