Erythropoietin (Epo) is the major regulator of the proliferation and differentiation of erythroid precursors through interaction with its receptor (Epo-R). Although Epo-R lacks a tyrosine kinase consensus sequence within its intracellular domain, the addition of its ligand to Epo-responsive cells, UT-7/Epo, induces the rapid and transient tyrosine phosphorylation of 145-, 130-, 80-, and 40-kDa cellular proteins. Tyrosine phosphorylation of these proteins occurred dose- and time-dependently. We showed that the tyrosine phosphorylated 145- kDa protein is identical to phospholipase C-gamma 1 (PLC-gamma 1). Tyrosine phosphorylation of this protein is detectable within 30 s and almost reaches the maximum at 1 min. This can last up to 10 min and declines thereafter. Additionally, in Epo-stimulated cells, PLC-gamma 1 become physically associated with 80- and 40-kDa proteins which have been tyrosine-phosphorylated in response to Epo. The activity of PLC-gamma 1 was also investigated using inositol 1,4,5-trisphosphate (Ins-P3) as an indicator. We found that stimulation of UT-7/Epo cells with Epo induces a significant accumulation of Ins-P3. This effect is dose-dependent and occurs very rapidly. The production of Ins-P3 can explain the Epo-induced mobilization of calcium from intracellular stores in these cells. These results demonstrate that Epo induces tyrosine phosphorylation and activation of PLC-gamma 1 to produce Ins-P3 and then it mobilizes calcium from intracellular stores. This signal transduction pathway may play a role in regulating the proliferation of erythroid cells.