Embryonal carcinoma (EC) cells are the malignant stem cells of teratocarcinoma and have the capacity to proliferate in the absence of serum growth factors. As yet no receptor protein tyrosine kinases have been identified on undifferentiated EC cells and as a consequence tyrosine kinase signaling pathways could not be studied in these cells. We have used stably transfected P19 embryonal carcinoma cells expressing a well-characterized receptor protein tyrosine kinase, the human epidermal growth factor receptor (hEGF-R) to study protein tyrosine kinase signaling mechanisms in undifferentiated EC cells. Here we report that the ectopically expressed hEGF-R contains EGF-inducible autophosphorylation activity and is rapidly internalized and degraded upon ligand binding. In addition, the exogenous hEGF-R confers EGF-responsiveness to these cells in that inositol phosphate formation and cytoplasmic-free Ca2+ concentration are enhanced in response to EGF. Furthermore, the Na+/H+ exchanger is activated in response to EGF, leading to a sustained rise in intracellular pH. Our results show that undifferentiated P19 EC cells contain the necessary components of protein tyrosine kinase signal transduction machinery.