U0126 inhibits H-ras.V12-induced Epo-independent growth of erythroid progenitors. Freshly isolated fetal liver TER119^- cells were infected with a bicistronic retrovirus encoding H-ras.V12 and hCD4 and cultured in fibronectin-coated wells in medium containing serum and Epo. U0126 (20 μM) was added into cells on day 1 and replaced every 24 hours. (A) The cells were harvested 6 hours after the addition of DMSO (control) or U0126. Expression of H-ras.V12 protein and activation of p44/42 were examined by Western blotting as in Figure 1. (B) The cells were cultured in fibronectin-coated wells in medium lacking Epo and in the absence or presence of U0126. Cell numbers are presented as total viable cells. Data are averages from triplicate cultures of a representative experiment and standard deviations were always less than 5% of the averages.