In vivo studies have shown that interleukin (IL)-1alpha binds to and is transported across brain endothelial cells, whereas in vitro studies have shown that brain endothelial cells respond to IL and contain mRNA for the IL-type 1 receptor. However, these binding sites have yet to be characterized. Herein, we used murine brain microvessels to characterize the binding of IL labeled with (125)I. Binding was temperature- and time-dependent with maximal binding after 4 h of incubation at 37 degrees C. The amount of radioactivity determined by HPLC to represent intact (125)I-labeled murine IL-1alpha at 4 h was approximately 100% in the incubation fluid and 80 to 90% for radioactive material recovered from the incubated cells. B(max) was 0.955 fmol and the K(d) was 292 pM for human (125)I-IL and binding was displaced by interleukin-1beta and interleukin-1 receptor antagonist but not by tumor necrosis factor alpha. Binding was dependent on magnesium and glucose. Incubation with antibodies showed that the binding site was not identical with the IL-type 1 receptor but closely resembled the blood-brain barrier transporter. These results show that murine brain endothelial cells have specific binding sites for IL and that these sites more closely resemble the transporter than the type 1 receptor.