Mouse alveolar macrophages (AM) were rendered tumoricidal after the intravenous administration of liposomes containing muramyl tripeptide phosphatidylethanolamine (MTP-PE), a lipophilic derivative of muramyl dipeptide. The addition of recombinant mouse interferon gamma (r-IFN-gamma) to the liposomes significantly potentiated this effect. This potentiation was also observed in therapeutic studies of mice bearing well-established spontaneous lung melanoma metastases. Multiple intravenous injections of liposomes containing both MTP-PE and r-IFN-gamma resulted in 70% survival in one group treated for small lung metastases and 50% in another group treated for large lung metastases. These data demonstrate that the presentation of r-IFN-gamma and MTP-PE in liposomes is more efficient in inducing the destruction of metastases than either agent administered alone.