The effect of systemic or local injection of Corynebacterium parvum at the tumor site on the growth of various murine tumors was studied in intact and congenitally athymic BALB/c mice. Systemic injection of C. parvum usually had a marked antitumor effect in both types of mouse. Two lymphomas, which regressed spontaneously in untreated intact mice but not in athymic mice, grew progressively in intact mice given systemic C. parvum, though their growth was inhibited in similarly treated athymic mice. Local injection into the site of the tumor markedly inhibited tumor growth in intact mice but was without effect in athymic mice. C. parvum was believed to exert its antitumor effects by two different mechanisms, only one of which was T-cell dependent. The mechanism not dependent on T-cells was particularly activated by systemic C. parvum injection.