A systematic study has been undertaken to analyze the spectrum of erythropoietic colonies obtained in cultures of human marrow cells plated in methyl cellulose. Colonies were identified as erythropoietic on the basis of the appearance in them of hemoglobin-containing erythroblasts. As found previously in mouse marrow cultures, three sequentially appearing types of colonies which differed in their ultimate cluster content could be readily distinguished. Small erythroid colonies containing 1-2 clusters reached a peak after 7-8 days; small bursts containing 3-8 clusters reached a peak after 10-12 days; and large bursts containing greater than 16 clusters reached a peak after 17-20 days. The previously reported enhancing effect of human leukocyte conditioned medium on burst formation seen in cultures of human nonadherent cells was found to be due largely to an effect on the formation of the largest, late appearing type of burst. By analogy with the mouse, the progenitors of such bursts would represent a primitive cell type which has a close relationship with pluripotent stem cells, as well as a second and independent close relationship to the progenitors of granulopoietic colonies.