In blood and marrow samples from ten normal adults, mixed colonies were found in agar cultures at concentrations of 4.1 +/- 3.4/10(5) mononuclear bone marrow cells, and of 4.7 +/- 4/5 X 10(5) mononuclear blood cells. Reseeding of five- to seven-day clusters of immature cells in fresh medium with 5% PHA leukocyte-conditioned medium (PHA-LCM) and 2 U erythropoietin (Epo) showed that 10(5) mononuclear cells contained as many as 42 +/- 12 CFU-mix in bone marrow and 12 +/- 7 CFU-mix in blood, i.e., ten times more than in the primary cultures. Attempts were made to discover why these potentially mixed colonies failed to develop in primary cultures. For this purpose five- to seven-day clusters of immature cells from cultures of 5 X 10(4) bone marrow cells grown with PHA-LCM, Epo, or both were reseeded in medium containing one or both stimulants. The presence of both in secondary culture gave the best CFU-mix growth, whatever the primary stimulation, i.e., more than one-third of the immature clusters gave rise to colonies of red cells and granulocyte lineages. The addition of one or both stimulants between days 7 and 14 of primary culture, however, did not increase the number of mixed colonies or give rise to late developing ones. Reseeding of colonies and clusters of 14-day primary cultures showed that at least one-third of erythroid bursts or of late-developing granulocyte colonies contained CFU-mix, but that less than 10% of 14-day clusters developed into mixed colonies. Cultures of nonadherent cells, mononuclear cells of less than 1.065/g density or with a velocity sedimentation rate of less than 6 mm/h gave the same results as the culture of total mononuclear cells; i.e., at least ten times more multipotent progenitors were found by reseeding after five to seven days than in primary cultures. We conclude that in human bone marrow and blood the physical and kinetic properties of pluripotent stem cells are similar to those of 14-day erythroid bursts and GM colonies. Under normal culture conditions, most of these cell types differentiate into one cell lineage only, and plurilineage differentiation is inhibited.