Five patients with advanced malignant melanoma, treated with viral oncolysate, had solitary central nervous system metastases that were removed surgically. Histologic examination revealed striking and significant mononuclear inflammatory cell infiltrates, consisting of a mean of 60% plasma cells and a lesser proportion of lymphocytes at the edges of the lesions, within their supporting fibrovascular trabeculae, and among the tumor cells. Comparable inflammatory changes were not found in solitary metastatic malignant melanomas removed surgically from the brains of 19 patients not treated with viral oncolysate. Similarly, multiple metastatic malignant melanomas obtained postmortem from the brains of 12 patients not treated with viral oncolysate showed minimal inflammatory responses. Ultrastructural examination of material from a single treated patient revealed morphologic abnormalities of the blood-brain barrier, changes that were perhaps conducive to infiltration of the neoplasm by inflammatory cells. The authors suggest that administration of viral oncolysate enhances the inflammatory cell response to metastatic malignant melanoma in the brain.