Bone marrow is the major site of hematopoiesis in the adult mammal. Bone marrow contains a highly organized microenvironment for the support of hematopoietic stem and progenitor cells, including the production of growth factors. Bone marrow cells also produce negative regulatory factors which may regulate hematopoiesis and inflammatory responses. In this paper we describe Reptimed, a unique bone marrow-derived factor with inhibitory activity for myelopoiesis and in vitro growth of myeloid cell lines. Reptimed was partially purified from bone marrow supernatants using a combination of solid-phase extraction and size exclusion chromatography. Reptimed is < 1000 Da MW and is water soluble. Reptimed inhibited growth of granulocyte-macrophage and macrophage colonies as well as proliferation of several myeloid leukemia cell lines. Reptimed may be part of a hemoregulatory circuit.