The growth of granulopoietic progenitors (CFU-C) and morphologically identifiable haemopoietic cells from rat bone marrow was studied in intraperitoneal diffusion chambers over a 6-day period. The aim of this study was to investigate whether stem cells preceding the granulopoietic progenitors are able to restore and to maintain depleted compartments of CFU-C and immature granulopoietic cells under the conditions of diffusion chamber culture. After treatment of rats with repeated doses of hydroxyurea (HU) a bone marrow cell suspension could be obtained wherein proliferating haemopoietic cells were reduced to a very low level and CFU-C to undetectable levels. During culture in diffusion chambers, growth of CFU-C, immature granulopoietic cells and macrophages could be observed from this HU-treated bone marrow. It is concluded that in diffusion chamber culture stem cells preceding CFU-C are able to rebuild functioning compartments of granulopoietic progenitors and morphologically identifiable haemopoietic cells.