A liquid culture method has been developed allowing for the in vitro growth of peripheral blood-derived hemopoietic progenitor cells of myeloid, erythroid, monocytic and megakaryocytic lineages. Adherent cell- and CD21-positive cell-depleted PBMC from normal subjects have been cultured in the presence of rhEPO, rhGM-CSF or rhIl-3. Culturing cells in liquid cultures and in plasma clots, a similar dose-response was observed for granulocytic cells/liquid culture and granulocytic colonies/plasma clot with rhGM-CSF, and also for erythroid cells/liquid culture and erythroid colonies/plasma clot with rhEPO. Comparing serum- liquid cultures to serum+ liquid cultures, the ratio of CD13+ cells to CD15+ cells was higher in serum- cultures, indicating a maturation arrest of myecloid cells with serum deprivation. Using dual-colour flow-cytometry, cell-cycle analysis of CD13+ cells, comparing the effects of rhGM-CSF to those of rhIl-3, have been performed. The liquid culture method promises to be a useful tool for the study of in vitro differentiation and proliferation of hemopoietic progenitor cells.