Current therapies for renal cell carcinoma have been limited by the unresponsiveness of metastatic disease to conventional treatments. Although the use of biological response modifiers as adjuvant therapy has generally not been successful against disseminated disease, in situ activation of macrophages to a tumoricidal state by liposome-encapsulated immunomodulators has been shown to eradicate metastatic cancer in murine tumor models. We, therefore, designed experiments to evaluate the ability of a new macrophage activator, CGP 31362, a synthetic bacterial cell wall analogue, to cause regression of spontaneous lung metastases in mice whose primary renal adenocarcinoma was removed by nephrectomy. Delivery of the CGP 31362 to the lungs was accomplished by its encapsulation in multilamellar phospholipid liposomes (MLV-CGP 31362). Therapy with repeated i.v. injections of MLV-CGP 31362 significantly reduced the number of lung metastases in nephrectomized mice. Therapeutic efficacy of MLV-CGP 31362 was influenced by the encapsulation ratio of CGP 31362 to total phospholipid, the dose of injected liposomes, and the frequency of administration. Optimal therapy was achieved by combining the use of i.v. MLV-CGP 31362 with the s.c. injection of recombinant murine gamma interferon. Administration of MLV-CGP 31362 prior to removal of the primary tumor and continuing postoperatively was superior to postoperative therapy alone. Several lines of evidence indicate that in situ activation of macrophages was responsible for the therapeutic effects of MLV-CGP 31362: (a) macrophages harvested from the lungs of treated mice had significant tumoricidal activity against cultured renal carcinoma cells, (b) activated macrophages, as defined by the MRP-14 marker, were present in lung tumor nodules of treated mice but not untreated mice, and (c) the in situ activation of alveolar macrophages was consistent with the in vivo deposition of 60% of radiolabeled MLV-CGP 31362 liposomes in the lungs following i.v. injection. The results reported here represent the first in vivo evaluation of MLV-CGP 31362 and offer additional evidence that macrophage combination with therapies that reduce tumor burden.