We have reported that human malignant glioma cell lines express high levels of plasma membrane interleukin-4 receptors (IL-4R). We have also reported that biopsy/surgical samples or primary explant cell cultures from brain tumors express mRNA and protein for the IL-4Ralpha chain, a primary IL-4-binding protein. However, whether IL-4R are expressed in brain tumors in situ has not been resolved. In addition, expression of IL-4R on the cell surface of various normal brain tissues is not known. We examined the expression of IL-4R by using a monoclonal antibody to the IL-4Ralpha chain (also known as IL-4R beta) in surgical/biopsy samples of brain tumor tissues by immunohistochemistry. Our data indicate that 15 of 18 glioblastoma multiforme (GBMs) tumors obtained from two different institutions and 12 other brain tumor samples are moderately to intensely positive for IL-4Ralpha. In contrast, although IL-4Ralpha mRNA was expressed, no IL-4R protein was detectable in two adult and one pediatric brain tissue specimens. In addition, a commercially available human neural tissue grid containing fixed tissues from various areas of brain showed no positive staining for the IL-4Ralpha chain. IL-4Ralpha expression was also demonstrated on astrocytoma grades I, II, and III. Because IL-4 cytotoxin comprised of a circularly permutated IL-4 and a mutated form of Pseudomonas exotoxin [IL4(38-37)-PE38KDEL] is cytotoxic to IL-4R-expressing cells, we tested whether primary GBM explant cell cultures are sensitive to IL-4 cytotoxin. Our data indicate that 13 of 15 GBM cell cultures were 25-74 times more sensitive to IL-4 cytotoxin compared with normal human astrocytes or the NT2 neuronal cell line. These observations indicate that human brain tumors in situ overexpress IL-4R compared with normal brain tissues, thus confirming our previous conclusions that IL-4R in brain tumors may serve as an attractive target for anticancer therapy.