Human interleukin (IL)-10 is a pleiotropic cytokine acting on a variety of immune cells. Here we show that the protein can be enzymatically iodinated to high specific radioactivity with retention of biological activity. The radiolabeled ligand binds specifically to its receptor in several mouse and human cell lines, notably human B-lymphoma line JY and mouse mast cell line MC/9. Human IL-10 apparently binds as a dimer to a single class of receptor in both the JY and MC/9 cell lines with a Kd in the 50-200 pM range. Interestingly, mouse IL-10 was capable of blocking binding of human IL-10 to mouse but not human cells. There appears to be at most only a few hundred IL-10 receptors/cell for both mouse and human cell lines examined. Chemical cross-linking of the radioiodinated hIL-10 to JY and MC/9 cells revealed a common protein complex with an apparent molecular mass of about 97 kDa. Additional high molecular weight complexes were detected with JY but not MC/9 cells.