We investigated the effects of granulocyte-macrophage colony- stimulating factor, interleukin-3, stem cell factor, interleukin-6, and granulocyte colony-stimulating factor (G-CSF) alone, and in combination, on the clonogenic potential of normal and aplastic anemia (AA) bone marrow mononuclear cells (BMMC and CD34+ cells. AA BMMC consistently produced a significantly lower absolute number of colonies than normal, but, when account was taken of the reduced proportion of CD34+ cells in AA BM, there was no significant difference in terms of cloning efficiency (CE). However, when removed from the influence of accessory cells, the CE of AA CD34+ cells decreased significantly more than normal, indicating a defect in their function, either in terms of dependence on accessory cell-derived factors or susceptibility to cell damage when sorted. Of the factors studied, G-CSF had the most significant effect on the response of CD34+ cells from both groups when removed from their accessory cells. This was particularly true for AA CD34+ cells, whose response to cytokine stimuli containing G-CSF enabled them to match the response of normal CD34+ cells.