Increased accumulation of erythroblasts in cocultures with macrophages occurs at all EPO concentrations. (A) Percentages of apoptotic (TUNEL-positive) cells at 44 hours after culture in various concentrations of EPO. In cocultures, apoptosis percentages for adherent and nonadherent erythroblasts were the same at all concentrations of EPO, and totals of both populations (■) are shown; control erythroblasts (○). (B) Fold increases of total viable erythroblasts in cocultures (adherent + nonadherent) (■) and viable control erythroblasts (○) from 6 hours to 44 hours at various EPO concentrations. Increased erythroblast accumulations in cocultures compared with controls were significant at all added EPO concentrations (*P < .05). (C) Erythroblasts at various times of culture were analyzed by TUNEL assays for percentage of apoptotic cells: adherent erythroblasts (■), nonadherent erythroblasts (♦), control erythroblasts (○). Results are the means plus or minus SE from 3 separate experiments in each part.