Antitumor activity induced by a heat-killed preparation of S. thermophilus against mouse fibrosarcoma was investigated. The treatment of Swiss mice with S. thermophilus prior to transplantation could not prevent tumors. However, animals cured by treatment with a S. thermophilus preparation failed to take up the tumor when rechallenged with fibrosarcoma. S. thermophilus did not induce antitumor activity in animals immunosuppressed by sublethal whole body gamma-irradiation (4 Gy) or hydrocortisone treatment prior to transplantation. Suppression of activity of macrophages by carrageenan had no effect on antitumor activity of the heat inactivated preparation of S. thermophilus. The intravenous administration of sera from cured animals was ineffective in curing the tumours. Spleen cells from cured animals could effectively transfer the antitumor activity to recipients transplanted with the tumor. This effect was abolished when the T-lymphocyte population in the inoculum was specifically depleted. The results thus suggest the involvement of T-lymphocytes in antitumor activity exhibited by S. thermophilus.