Antitumor activity of Leuconostoc mesenteroides, Streptococcus cremoris and Streptococcus lactis was investigated in solid mouse fibrosarcoma. Intratumor administration of lyophilised bacterial cells at a dose of 20 mg/kg body wt resulted in the regression of tumors in a maximum number of animals. Intraperitoneal injection at the same dose resulted in a significant increase in the survival time but was ineffective in curing the animals. Intratumor injection of Streptococcus thermophilus at a dose of 20 mg/kg body wt also inhibited the growth of fibrosarcoma accompanied by an increase in the survival time while intraperitoneal administration of S. thermophilus prolonged only the survival time and did not result in cure. In mice bearing the ascitic form of sarcoma-180 or Ehrlich ascites carcinoma, intraperitoneal administration of S. thermophilus resulted in complete cure in a very significant proportion of tumor-bearing mice. S. thermophilus was more effective in sarcoma-180 than in Ehrlich ascites carcinoma. The cured tumors did not recur.