Single and multiple doses of Corynebacterium parvum (C. parvum) ranging from 0.1--60 mg/kg were tested for antitumor activity against 10(6) sarcoma 180 cells in male CD1 mice. Determinations were made of the optimal dose and time of treatment needed to produce maximum suppression of the tumor using both median survival time and percent survival to day 90 as endpoints. A dose of 1 mg/kg given 3 days before sarcoma 180 transplant produced complete protection (100% survival). All other treatment regimens produced less of an effect. Single doses of 1, 10 and 60 mg/kg had significant antitumor activity when administered either on day 3, 2, or 1 before tumor implant, 0.1 mg/kg protected only when given on day 3. All single doses given 5 or 8 days before and anytime after tumor were ineffective. Multiple doses were only of advantage over single doses when treatments were after tumor cell inoculation. In vitro cytotoxicity studies demonstrated that both 1 and 60 mg/kg enhanced tumor cell killing by cells isolated from the peritoneal cavity, with the 1 mg/kg dose producing a greater effect. It was concluded that dose and time of administration of C. parvum, in relation to tumor implant, were important in determining optimal antitumor activity against sarcoma 180.