Single and multiple doses of Corynebacterium parvum ranging from 0.01 to 60 mg/kg as the total dose were tested at selected times against 10(6) P388 leukemia cells inoculated ip in BDF1 mice. The minimum dose needed for maximum protection was determined according to the ability of the treatment to produce an increase in the lifespan (ILS) of mice. The ip dose required after transplant for significant ILS was 40 mg/kg as the total dose. The administration of a single dose 2 days prior to implant produced maximum antitumor effects at 1 mg/kg, with all other doses above and below this level being significantly less effective. Protection was achieved before implant, with the following maximum ILS recorded for each dose and time: 1 mg/kg on Day -2 = 110%; 60 mg/kg on Day -5 = 70%; 10 mg/kg on Day -2 = 50%; and 0.1 and 0.01 mg/kg on either Day -2 or Day -5 = 25%-30%. Cytotoxicity studies demonstrated that 51Cr release after 1 mg/kg was more rapid in onset (peaking on Day 3 after treatment) than after 60 mg/kg (peaking on Day 5 after treatment). It was concluded that the proper adjustment of the dose and time of administration of C. parvum prior to tumor transplant enhanced antitumor activity of this immunopotentiator. Considerably higher doses were required after transplant for the desired effects to be achieved. The cytotoxicity studies demonstrated differences in tumor cell-killing which were dose-related and may be a contributing factor in the dose and time-related antitumor effects of C. parvum.