Systemic cytokine therapy in cancer has major side effects, and we reasoned that the local infusion of cytokines into tumors could induce local immunologic responses with minimal toxicity and potentially strong systemic anticancer effects. This study investigated the toxicity and effectiveness of intralesional granulocyte/macrophage-colony-stimulating factor (GM-CSF) infusion in solid-tumor masses. We studied 14 patients (12 men, two women) with malignant mesothelioma (MM), aged 60 years (range, 46-70 years), with stage 2 disease, in whom the tumor was of sufficient size and accessibility for an intralesional catheter to be inserted. Recombinant human GM-CSF (Molgramostim; Schering Plough) was infused intralesionally for 8 weeks, by using a portable pump, at a dose of 2.5-10 micrograms/kg/day. One patient using GM-CSF developed histologically confirmed necrosis of tumor surrounding the distal catheter, one developed a marked lymphocytic infiltrate in the tumor and had a partial response measured by chest computed tomography (CT) scan, 10 progressed, and three had no response. Neutrophilia with morphologic evidence of neutrophil activation and clinical features suggestive of neutrophil plugging of blood vessels occurred at doses > 5 micrograms/kg/day. In vitro, GM-CSF doubled human neutrophil/CD11b/CD18 expression, suggesting that neutrophil clumping as seen in vivo might be due to integrin upregulation. Intralesional infusion of cytokines is feasible but can be associated with systemic toxicity and has considerable technical problems. It produces a localized immune reaction with tumor regression in a minority of patients.