Human glioblastoma but not normal brain cells express numerous receptors for the cytokine interleukin (IL)-4. To target these receptors, we have investigated the safety and activity of directly infusing IL-4(38-37)-PE38KDEL, a chimeric protein composed of circularly permuted IL-4 and a truncated form of Pseudomonas exotoxin (PE), into recurrent malignant high-grade gliomas. IL-4(38-37)-PE38KDEL (IL-4-toxin) was infused over a 4-8-day period into gliomas of nine patients by one to three stereotactically placed catheters. No apparent systemic toxicity occurred in any patient. The infusion of IL-4-toxin in six of nine patients showed glioma necrosis as evidenced by diminished gadolinium enhancement on magnetic resonance imaging. Seven of nine patients underwent craniotomy because of increased intracranial pressure at 16-101 days after the beginning of infusion. In six of these seven patients, partial-to-extensive tumor necrosis with edema was confirmed pathologically. No histological evidence of neurotoxicity to normal brain was identified in any patient. Two patients were not operated on; by magnetic resonance imaging, one showed mottled gadolinium enhancement, and the other showed extensive necrosis of tumor leading to complete remission; this patient remains disease-free > 18 months after the procedure. We conclude that direct glioma injection of IL-4(38-37)-PE38KDEL is safe without systemic toxicity. Local toxicity seemed attributable mainly to tumor necrosis or occasionally to the volume of infusion. Histological evidence of toxicity to normal brain was not observed and in many patients, could be pathologically excluded. Additional patients are being treated to determine the maximal tolerated concentration and volume of IL-4(38-37)-PE38KDEL.