The inhibitory effect of thrombin on Epo-dependent DNA synthesis is reversed by Botulinus C3 exotoxin (C3). Progenitor cell cultures were growth factor–starved for 16 hours in the presence or absence of C3 toxin (30 to 40 μg/mL). At the end of this period, all cultures received Epo (0.5 U/mL). Thrombin was added as indicated in the column legends. The data are normalized with respect to the effect of Epo alone and give the mean ± SEM of nine different cultures from three batches of cells (##P < .01; **P < .01, paired test). Note that addition of C3 toxin by itself caused an 11% ± 3.7% decrease of Epo-stimulated DNA synthesis. See footnote in text for a comment on the low inhibitory efficacy of thrombin in this group of experiments.