Following low-dose cyclophosphamide (CY) therapy (15 mg/kg) of mice bearing a large MOPC-315 tumor, the suppressive activity of their Sephadex G-10-adherent spleen cells (primarily macrophages) is overcome. Accordingly, when Sephadex G-10-adherent spleen cells from CY-treated tumor-bearing mice are added to the in vitro immunization culture of normal spleen cells, they do not suppress but actually bring about the generation of an augmented level of antitumor cytotoxicity. The ability to enhance the generation of antitumor cytotoxicity appears in the Sephadex G-10-adherent spleen cell population by Day 5 post-CY therapy of tumor-bearing mice and persists for at least 55 days; no such immunopotentiation is observed following administration of a low dose of CY to normal mice. In order for the immunopotentiating cells from CY-treated tumor-bearing mice to be effective in enhancing the generation of antitumor cytotoxicity, they must be added to the immunization culture of normal spleen cells no later than Day 3 (out of the 5 days) post-culture initiation. The CY-induced immunopotentiating activity resides in the T-cells, as is evident from the following observations. The immunopotentiating activity was abolished when the Sephadex G-10-adherent spleen cell population from CY-treated tumor-bearing mice was depleted of T-cells by anti-Thy 1.2 plus complement but not when this adherent spleen cell population was depleted of macrophages by carbonyl iron and magnet. Moreover, the immunopotentiating activity was also present in a population of CY-treated tumor-bearer spleen cells highly enriched for T-cells by passage through nylon wool columns. Thus, low-dose CY therapy overcomes the immunosuppressive activity of macrophages and induces the appearance of T-cell-mediated immunopotentiating activity, thereby leading to the development of an augmented level of antitumor cytotoxicity that can cooperate effectively with the tumoricidal activity of CY in the eradication of a late-stage, large s.c. tumor and extensive metastases.