Different models have been proposed to explain lineage commitment in hemopoiesis. Some suggest that lineage commitment occurs in a stochastic manner without the direct influence of extracellular factors; others postulate that cytokines determine whether multipotent cells will become erythroid or granulocyte/macrophage progenitors. In the present study, the patterns of proliferation and differentiation of individually sorted human cord blood-derived primitive hemopoietic cells (highly enriched for multipotent progenitors) were analyzed in a serum-free culture system supplemented with different cytokine combinations. In a first set of experiments, the response of individual cells to different cytokine combinations was compared, whereas in a second set of experiments, single cells were allowed to undergo one division after which the two daughter cells were physically separated and cultured in either the same or different cytokine combinations. Proliferation of progenitor cells was absolutely dependent on cytokines, and the combination of mast cell growth factor plus interleukin 6 was sufficient to induce mitosis. When cytokine combinations favoring erythropoiesis and/or myelopoiesis were added to the cultures, a more vigorous proliferative response of the sorted primitive progenitors was observed. Interestingly, the relative proportions of granulocyte/macrophage, erythroid, and multipotent progenitors remained more or less the same regardless of the cytokine combination used, indicating a permissive rather than an instructive role for cytokines in hemopoietic differentiation. Asymmetric cell divisions, defined as a division that yields two daughter cells with distinct functional properties, were observed in 3-17% of the progenitor cells capable of forming colonies under our experimental conditions. In the rest, symmetric divisions involving multipotent and lineage-committed progenitors were observed. The results of this study demonstrate that the asymmetric cell divisions that occur in the early stages of hemopoiesis at the level of multipotent progenitors cannot be skewed by the addition of specific cytokine combinations. These findings support the hypothesis that lineage commitment in hemopoiesis occurs in a stochastic manner by mechanisms that remain to be elucidated.