Effect of Epo in the absence and presence of thrombin, SCF, and PMA on cytosolic PKC activity in human erythroid progenitors. Cells were pretreated for 1 hour with Epo (0.5 to 1 U/mL), with Epo (0.5 U/mL) and thrombin (2 U/mL), with SCF (50 ng/mL), or with PMA (10 nmol/L). Cytosolic PKC was immunoprecipitated with a monoclonal antibody. Enzymatic activity of the precipitate was estimated by measuring phosphate transfer to the substrate histone III-SS. Immunoprecipitates of untreated cells served as controls. Epo with or without thrombin as well as SCF increased, while PMA decreased immunoprecipitated PKC activity. Significance of differences to controls was checked by ANOVA followed by Dunnett’s test (*P< .05; **P < .01). Values give the mean ± SEM from five to nine separate assays from four to seven different batches of cells, except for PMA where the mean from three different cell preparations is given. Basal kinase activity in the absence of cofactors, but in the presence of EGTA, is documented for each condition by the height of the lightly shaded segment at the base of each column.