A precipitable cytoplasmic fraction of Propionibacterium acnes (C. parvum) has been isolated and shown to have antitumor activity in a new mouse model. The model was designed to determine the effect of intralesional treatment in the left rear foot pad on the growth of P815 mastocytoma already growing in both rear foot pads. Whole cells of P. acnes inhibited growth of the intralesionally injected foot pad and slowed growth of the contralateral foot pad (distant primary lesion). The cytoplasmic fraction (50K fraction) inhibited both foot pad tumors equally when injected into the left foot pad only. These results indicate that the mouse response is systemic and not local and that the fraction is a more effective antitumor treatment than the whole cell. The purification and characterization of the 50K fraction is one of the principal goals of the projected study. Examination of the mechanism of action of both P. acnes whole cells and 50K fraction is the second major goal. A well characterized, immunopotentiating moiety with a known mechanism of action should become an effective addition to present anticancer therapies.