In vitro activity of Epo produced by P19 cells. Epo activity was assayed with Epo-dependent proliferation of Ep-FDC-P2 cells measuring the increased absorbance at 600 nm due to reduction of MTT. (▴, ▵) P19 Epo; (•, ○) mouse serum Epo; (▪, □) human recombinant Epo; (▵, ○, □) in the presence of sEpoR capable of binding with Epo. When assayed in the presence of sEpoR, Epo was preincubated with sEpoR (100 μg/mL) for 1 hour. Each point is the mean of duplicate assays.