We have cloned a cDNA encoding a receptor identical to the native Mr 80,000 glycoprotein that binds interleukin (IL) 1 alpha and -beta in murine T cells. Chinese hamster ovary (CHO) cells transfected with this T-cell IL-1 receptor (IL-1R) [CHO(IL-1R)] cDNA express approximately 100,000 IL-1Rs per cell, compared to the less than 100 receptors present on control CHO cells. For two functional responses to IL-1, prostaglandin synthesis and cytokine secretion, CHO(IL-1R) cells were 1000 times more sensitive to IL-1 alpha than were control CHO cells. Northern blot analysis and antibody precipitation demonstrated that one of the cytokines induced was granulocyte colony-stimulating factor and that mRNA levels for this cytokine were increased in CHO(IL-1R) cells by IL-1 alpha concentrations that had no effect on control cells. To establish the role of the recombinant receptors in signal transduction, an IL-1R cDNA modified by deletion of the predicted cytoplasmic domain was expressed in the CHO cell line termed CHO(IL-1R delta CT). CHO(IL-1R delta CT) cells expressed approximately 100,000 high-affinity IL-1 binding sites per cell, but these cells were less sensitive than control lines to IL-1, as measured by prostaglandin and cytokine release. These results show that the IL-1R cDNA encodes the entire functional receptor and that the cytoplasmic domain is required for signal transduction but not ligand binding.