Tyrosine phosphorylation induced by activation of receptors for growth factors and cytokines. Erythroid progenitors were withdrawn from growth factors overnight and subsequently stimulated for 10 minutes with the cytokines indicated, using 10-fold higher concentrations than used in liquid cultures. Cell lysates were separated on acrylamide gels and subjected to Western blot analysis using the antiphosphotyrosine antibody 4G10. Arrows indicate the position of the size markers used.