Using FDC-P1 cells stably transfected with a murine erythropoietin receptor cDNA as a model, we recently have shown that erythropoietin (EPO), IL-3 and GM-CSF each induce the rapid phosphorylation of a common cytosolic target, i.e., a M(r) 100,000 phosphoprotein "pp100". Presently, we demonstrate that cytokine-induced phosphorylation of pp100 is primarily at tyrosine residues. This is shown by Western blotting with the anti-phosphotyrosine antibody PY20, and by the resistance of [32P]-pp100 to hydroxide-mediated hydrolysis of phosphates. These data, together with the recent observation by Linnekin et al. that pp100/p97 apparently associates directly with EPO receptors, suggest that pp100 may comprise an immediate common component in the signal transduction pathways of EPO, IL-3, GM-CSF and possibly other type I/II cytokine receptors. Additional analyses suggest that pp100 is distinct from a previously described M(r) 100,000 cytosolic target which is tyrosine phosphorylated in hematopoietic cells upon activation of T-cell receptors.