To determine the extent accessory cells mediate the effects of recombinant human granulocyte-macrophage colony-stimulating factor (rhGM-CSF) on human hemopoietic progenitors in vitro, we added this hemopoietin to liquid cultures of single CD34-positive marrow cells. These were selected on a fluorescence-activated cell sorter using the HPCA-1 (My10) antibody. Myeloid, erythroid and a few mixed clones developed in 13% of wells in the apparent absence of accessory cells at the beginning of culture. Although accessory cells were generated quickly from the myeloid progenitors and could have mediated the action of rhGM-CSF, this was not the case in the majority of the erythroid clones in which no other cell types were recorded. We conclude that rhGM-CSF can act directly on a subset of erythroid progenitors and probably induces a substantial number of myeloid clones directly.