In order to study whether cytopathic HIV1 infection of haemopoietic progenitor cells is involved in the derangement of haemopoiesis in patients with HIV1 infection, we infected enriched progenitor cells with HIV1, by addition of viral inoculate supernatants from HIV1-infected peripheral blood mononuclear cells or by coculture with HIV1-infected monocytes/macrophages. Progenitor cells were seeded into colony assays and single colonies were chosen for HIV1 mRNA determination by in situ hybridization. Growth of progenitors was not affected by infection. However, up to 42% of colonies of pluripotent progenitor cells (colony-forming unit/granulocyte-erythrocyte-monocyte; CFU-GEM) and committed progenitor cells CFU/granulocyte-monocyte (CFM-GM) contained HIV1 mRNA-expressing cells. In addition, we studied HIV1 infection of progenitor cells from the bone marrow of 6 patients with AIDS or AIDS-related complex. Two patients were negative, two had a few colonies expressing HIV1 mRNA in a minority of cells, and in the remaining two, up to 11% of CFU-GM contained HIV1-expressing cells. Thus, infection of progenitor cells with HIV1 was achieved experimentally in vitro and occurs in vivo. However, growth of progenitors after in vitro infection continues and therefore HIV1 infection does not seem to contribute directly to the reduced incidence of haemopoietic progenitor cells in vivo.