The ratio of tyrosine-phosphorylated EGF-receptors was estimated for surface and endosomal EGF-receptor complexes on A431 cells. The cells were incubated with 125I-EGF, and then 125I-EGF was covalently bound to the receptor by cross-linking reagent-suberate. Following the cell lysis, the tyrosine-phosphorylated receptors were immunoprecipitated by polyclonal antiphosphotyrosine antibody. The ratio between the whole pool of 125I-EGF-linked and tyrosine-phosphorylated receptors were determined by densitometry of gels. It is found that only 20-45% of EGF-linked receptors are tyrosine-phosphorylated. The same share of tyrosine-phosphorylation was found for internalized EGF-receptor complexes.