Stimulation of DNA synthesis in human erythroid progenitors by various growth factors. Acid-precipitable³H-thymidine incorporation was measured 24 hours after addition of growth factors to cells maintained for 16 hours in growth factor–depleted medium (‘starved’). Basal thymidine incorporation in the absence of added growth factors is indicated by the dashed line. The columns give mean values ± SEM from six separate cultures, except for PMA/Epo, which is the mean of three cultures. Statistical differences between group means was tested by one-way ANOVA followed by Bonferroni’s test to compare individual pairs of columns labeled **, ##, $$, &&. All tested differences were significant on the P< .001 level.