The mAb CC11 and CB5 reacted against all 18 IL-3-dependent cell lines tested, but not against cells insensitive to IL-3. Up to 53% nucleated cells from fetal liver (14th day of gestation) and 79% bone marrow cells of young adult mice were positive for both CC11 and CB5 antigens, but cells from thymus, lymph node, heart, and kidney were negative. The molecule recognized by both antibodies has an Mr of 50,000-70,000, a pI of 5.7-6.2, and carries heterogeneous N-linked glycans of high Mr. Both CC11 and CB5 specifically inhibited the growth of clones supported by rIL-3. Neither antibody affected the action of IL-1, IL-2, or B cell maturation factor; the proliferative responses of splenocytes to Con A, PWM, and LPS; nor the maturation of spleen B cells into antibody-secreting cells stimulated by LPS. rIL-3 specifically modulated the expression of the CC11/CB5 glycoprotein on the cell membrane of IL-3-dependent clones. Finally, freshly isolated bone marrow cells that have the CC11/CB5 glycoprotein on the cell membrane proliferated in response to IL-3, whereas cells that lack this molecule did not. We suggest that CC11 and CB5 react against receptors for mouse IL-3.