To determine the role of natural killer (NK) cells in the regulation of human erythropoiesis, we studied the effects of NK-enriched cell populations on the in vitro proliferation of erythroid stem cells at three different levels of maturation (day 14 blood BFU-E, day 5–6 marrow CFU-E, and day 10–12 marrow BFU-E). NK cells were enriched from blood by Percoll density gradient centrifugation and by fluorescence- activated cell sorting (FACS), using the human natural killer cell monoclonal antibody, HNK-1. The isolated enriched fractions were cocultured with autologous nonadherent marrow cells or blood null cells and erythropoietin in a methylcellulose erythroid culture system. Cells from low-density Percoll fractions (NK-enriched cells) were predominantly large granular lymphocytes with cytotoxic activity against K562 targets 6–10-fold greater than cells obtained from high- density Percoll fractions (NK-depleted cells). In coculture with marrow nonadherent cells (NA) at NK:NA ratios of 2:1, NK-enriched cells suppressed day 5–6 CFU-E to 62% (p less than 0.025) of controls, whereas NK-depleted cells slightly augmented CFU-E to 130% of controls (p greater than 0.05). In contrast, no suppression of day 10–12 marrow BFU-E was observed employing NK-enriched cells. The NK CFU-E suppressor effects were abolished by complement-mediated lysis of NK-enriched cells with the natural killer cell antibody, HNK-1. Highly purified HNK- 1+ cells separated by FACS suppressed marrow CFU-E to 34% (p less than 0.025) and marrow BFU-E to 41% (p less than 0.025) of controls. HNK- cells had no significant effect on either BFU-E or CFU-E growth. NK- enriched cells were poor stimulators of day 14 blood BFU-E in comparison to equal numbers of NK-depleted cells or T cells isolated by E-rosetting (p less than 0.01). Interferon boosting of NK-enriched cells abolished their suboptimal burst-promoting effects and augmented their CFU-E suppressor effects. These studies provide evidence for a potential regulatory role of NK cells in erythropoiesis. The NK suppressor effect is maximal at the level of the mature erythroid stem cell CFU-E. These findings may explain some hypoproliferative anemias that develop in certain NK cell-activated states.