The purpose of these studies was to determine whether blood monocytes of patients with different stages of colorectal carcinoma could be activated by various immunomodulators to become tumor cytolytic. Monocytes obtained from 12 colorectal carcinoma patients and 8 normal donors were incubated in vitro with free or liposome-encapsulated agents. The cytotoxic properties of the monocytes were determined subsequent to interaction with radioactively labeled allogeneic colon carcinoma cells, melanoma cells, glioblastoma cells, and allogeneic nontumorigenic skin cells. Blood monocytes from normal donors and all colorectal carcinoma patients were activated in vitro to become tumoricidal by immunomodulators in free form or entrapped within liposomes; i.e., the monocytes recognized and lysed tumorigenic cells but not nontumorigenic cells. The tumoricidal activity of monocytes was observed in blood monocytes obtained from patients even after multiple doses of radiotherapy and chemotherapy, and that fact suggests that the in vivo activation of macrophages may be feasible.