Expansion of mature neutrophils has been observed in mice lacking the murine interleukin (IL) 8 receptor homolog [mIL-8Rh(-/-)], and human (hu) IL-8 suppresses proliferation of primitive myeloid cells in vitro and in vivo. To evaluate involvement and relevance of murine IL-8 receptor homolog (mIL-8Rh) in negative regulation of myelopoiesis, we studied mIL-8Rh(-/-) and (+/+) mice raised in a normal or germ-free environment. Immature myeloid progenitors from mIL-8Rh(+/+) mice bred under normal or germ-free conditions were significantly suppressed in vitro by recombinant huIL-8, macrophage inflammatory protein (MIP)-1 alpha, platelet factor (PF) 4, interferon inducible protein (IP) 10, monocyte chemotactic peptide (MCP) 1, and H-ferritin. In contrast, progenitors from mIL-8Rh(-/-) mice were insensitive to inhibition by IL-8, but not to these other chemokines and H-ferritin. Mouse MIP-2, a ligand for mIL-8Rh, suppressed progenitors from normal but not mIL-8Rh(-/-) mice. Under normal environmental conditions, enhanced numbers of myeloid progenitors were found in femur, spleen, and blood of mIL-8Rh(-/-) compared with mIL-8Rh(+/+) mice. Numbers of myeloid progenitors were greatly decreased in mIL-8Rh(-/-)and (+/+) mice in germ-free conditions, and were either not significantly enhanced in mIL-8Rh(-/-) mice compared with (+/+) mice or were only moderately so. Differences in progenitors/organ between a germ-free and normal environment were greater for the mIL-8Rh(-/-) mice. These results document selective insensitivity of myeloid progenitor cells from mIL-8Rh(-/-) mice to inhibition by huIL-8 and mouse MIP-2 and a large expansion of myeloid progenitors in these mice, the latter effect being environmentally inducible. This provides strong support for a negative myeloid regulatory role played by the mIL-8Rh in vivo, whose active ligand may be MIP-2.