&NA;Adoptive immunotherapy utilizing human recombinant interleukin 2 (rIL‐2) in conjunction with lymphokine‐activated killer cells has shown some efficacy in the treatment of various types of cancers, particularly renal carcinoma and melanoma. Intravenous administration of rIL‐2, with or without lymphokine‐activated killer cells, produces a variety of serious side effects and approximately one‐third of the patients experience a decrease in neurological status. Our previous investigations in animals have indicated that a single intravenous injection of rIL‐2 can compromise the integrity of the blood‐brain barrier (BBB). The present study examined the histopathological effect and BBB changes in rats which occur after a single intracerebral injection of rIL‐2, its excipient, or saline into the parietal lobe. Animals were killed at various intervals up to 8 days (1 hour after intravenous injection of horseradish peroxidase), and the brain tissue was sectioned and processed for light microscopy. All animals showed increased cerebrovascular permeability for horseradish peroxidase due to traumatic BBB disruption at 4, 12, and 24 hours after injection. Extravasation of horseradish peroxidase persisted at 3 and 8 days only in animals injected with rIL‐2. Injections of rIL‐2 led to an increased leukocytic infiltration into the injection site, perivascular cuffing, and localized edema by 24 hours, which continued to increase over the 8‐day study period. These results suggest that a single injection of human rIL‐2 into the brain of rats induces an influx of leukocytes into the brain and may contribute to the cellular events that perpetuate a trauma‐induced compromise in the integrity of the BBB. (Neurosurgery25:202‐208, 1989)