The interleukin-1 (IL-1) receptor on rat bone marrow cells was investigated using 125I-labeled IL-1 alpha and -1 beta. These radiolabeled ligands bound to the rat bone marrow cells in a specific and saturable manner with Kd values of 0.36 +/- 0.072 nM and 1.9 +/- 0.27 nM, respectively. In a competitive binding experiment, IL-1 alpha and -1 beta inhibited the binding of 125I-IL-1 alpha with Ki values of 0.35 +/- 0.041 nM and 2.9 +/- 1.0 nM. The binding of 125I-IL-1 beta was inhibited by IL-1 alpha and -1 beta with Ki values of 0.27 +/- 0.020 nM and 0.74 +/- 0.12 nM. In cross-linking experiments, 125I-IL-1 alpha was covalently incorporated into two proteins of 163 kDa and 63 kDa. These results suggested the presence of two binding proteins for IL-1 on the rat bone marrow cells.