Interleukin 1 beta (IL-1 beta), one of two different polypeptide hormones with interleukin 1 (IL-1) biological activity, produced by activated human monocytes, is a 17.5-kDa protein. IL-1 beta binds specifically to a variety of cells; the cellular distribution of binding is consistent with reported biological responsiveness. In this report we show that two unrelated, but IL-1-responsive, cell lines, LBRM-33-1A5, a T-lymphoma line, and BALB/3T3, a fibroblast line, bind 125I-labeled IL-1 beta via similar plasma membrane receptor molecules. The T-lymphoma cells possess 238 +/- 16 plasma membrane receptors per cell and bind 125I-labeled IL-1 beta with an affinity of 3.6 +/- 0.9 X 10(9) M-1. The IL-1 receptor has a molecular size of approximately equal to 79.5 kDa, as estimated by affinity cross-linking. The fibroblasts possess 4.8 +/- 0.5 X 10(3) IL-1 receptors per cell and bind 125I-labeled IL-1 beta with an affinity of 2.6 +/- 0.5 X 10(9) M-1. The molecular size of the receptor molecule on the fibroblasts is approximately equal to 78 kDa. Despite the similarity in the characteristics of the ligand-receptor system on the two different cell types, the biological responses of the two cell types to IL-1 beta occur at IL-1 beta concentrations that differ by four orders of magnitude.