The characteristics of PBSC mobilized with GM-CSF, which has been shown to augment monocyte/macrophage (Mo/Mx) antitumor and accessory activities, were evaluated. Patients with metastatic cancers were treated with GM-CSF at 5 micrograms/kg sc, days 1 to 7; leukaphereses were performed on days 6 and 7. A mean of 3.3 x 10(10) mononuclear cells were collected, 59% of which were lymphoid and 32%, monocytoid. Spontaneous Mo/Mx tumor cell cytotoxicity was not detectable in the leukapheresis product, either before or after cryopreservation; Mo/Mx tumor cell cytotoxicity, however, was inducible in vitro with IFN-gamma. Likewise, spontaneous lymphocyte cytotoxicity was not detectable in the leukapheresis product; lymphokine-activated killer cell activity was inducible in vitro with IL-2. Whereas lymphoproliferative responses to tetanus toxoid of cryopreserved PBSC were less than that of freshly collected PBSC, the capacity of Mo/Mx from cryopreserved PBSC to function as accessory cells in the lymphoproliferative response was maintained. These results indicate that significant numbers of immune cells can be mobilized with GM-CSF alone. Cryopreserved, GM-CSF-mobilized PBSC do not demonstrate spontaneous antitumor cytolytic activity; however, accessory activity is present and antitumor cytolytic activity mediated by both monocytoid and lymphoid cells is inducible.