Although binding sites for IL-1 have been identified in the mouse brain, it is still unknown whether these binding sites correspond to the type I or type II IL-1 receptor. Quantitative autoradiography was used to confirm the presence of specific binding sites for radiolabelled recombinant human IL-1 alpha (125I-HuIL-1 alpha) in the brain of DBA/2 mice. IL-1 binding was highest in the dentate gyrus, consisting of a single class of high affinity binding sites with a Kd of 0.1 nM and a Bmax of 57 fmol/mg protein. A similar Kd of 0.2 nM was obtained using isolated membranes from the whole hippocampus, although the number of binding sites was lower (2 fmol/mg protein). Affinity cross-linking of 125I-Hu-IL-1 alpha to hippocampal membranes revealed the existence of two types of IL-1 receptor proteins, consistent with the sizes of the type I (85 kD) and type II (60 kD) IL-1 receptor. Oligonucleotide probes were then synthesized and used in RT-PCR followed by Southern blotting to show that the whole brain expresses transcripts for both the type I and type II IL-1 receptors. The murine neuroblastoma cell line, C1300, expresses type I rather than type II IL-1 receptor mRNA. The type I receptor protein can be identified by flow cytometry on the membrane of the C1300 neuronal cell line using indirect immunofluorescence with a rat anti-mouse type I IL-1 receptor MoAb. These data show that mouse brain expresses both type I and type II IL-1 receptor mRNA and proteins and offer further support to the idea that type I IL-1 receptors are synthesized and expressed by neurons.