mAb to murine IL-5R were prepared by means of fusion between mouse myeloma cells and spleen cells from a rat immunized with membrane-enriched fractions of IL-5-dependent early B cell line (T88-M). Two mAb (H7 and T21) were selected for their competitive inhibition of receptor binding by 35S-labeled IL-5 and of IL-5 biologic activities. The number of binding sites recognized by the mAb on different cell lines correlated with IL-5 responsiveness. Most surface IgM+ peritoneal B cells were H7+ and more than 70% were also Ly-1(CD5)dull+, and responded to IL-5 for polyclonal IgM production in a high frequency. A significant proportion of splenic B cells reacted with these mAb, although lower number (one-log less) than peritoneal B cells and a small proportion of H7dull+ splenic B cells seems to be Ly-1(CD5)dull+, 1 of 200 splenic B cells responded to IL-5 for IgM production. These results suggest that IL-5R+ B cells may consist of a subpopulation of B cells. Intriguingly, lymphoid populations of bone marrow cells were stained with H7 and T21, whereas myeloid populations were brightly stained with only T21. Finally, both H7 and T21 mAb specifically precipitated a protein of a Mr 60,000 from 125I-labeled cell lysates of IL-5R+ T88-M cells. The IL-5R with similar size (Mr 55,000 to 60,000) was precipitated from the cell lysates of peritoneal B cells. T21 mAb but not H7 mAb precipitated a protein of a Mr 110,000 from the cell lysates of bone marrow cells.