We conducted a pilot study using liposome-encapsulated muramyl tripeptide phosphatidylethanolamine (L-MTP-PE) preoperatively in patients with stage III or IV resectable melanoma who were at high risk for recurrence. Patients received L-MTP-PE for 1 month before surgery and then 5 months postoperatively. Several immune parameters were monitored during preoperative therapy to search for correlations with clinical (tumor) response. The 18 patients were classified into three groups according to their responses and disease-free intervals: no evidence of disease (NED) at week 24 of therapy, relapse during therapy and progressive disease on therapy noted at the time of surgery. Six of nine patients in the NED group demonstrated increased monocyte tumoricidal activity (MTA) during week 1 of therapy. MTA increased in three of the six patients in the relapse group. MTA did not increase in the three patients who had progressive disease on therapy. Plasma neopterin levels were elevated by 72 h following the first L-MTP-PE dose in all 18 patients. Circulating levels of tumor necrosis factor were elevated in 15 of 16 patients tested, and IL-6 levels were elevated in all 18 patients. Melanoma cells from all three patients with progressive disease at the time of surgery proliferated well in vitro, whereas tumor cells from 10 of the 15 patients in the other two groups did not proliferate. There were no discernible differences among the three groups in the magnitude of IL-2-induced proliferation of tumor infiltrating lymphocytes. However, IL-2-activated TILs from the NED group exhibited cytotoxicity against autologous tumor cells in vitro. In summary, whereas L-MTP-PE stimulated several immunologic responses in all patients, the only two parameters that correlated with clinical status were MTA and the tumor proliferation assay. These two biologic assays could serve to distinguish potential responders from nonresponders early in the course of treatment.