Protein kinases associated with the transforming proteins of a number of retroviruses are specific for tyrosine. Several proteins in cells transformed by these viruses are phosphorylated at tyrosine. We have now identified three unrelated abundant nonphosphorylated cellular proteins of 46,000, 39,000 and 28,000 daltons in chick embryo cells, which are the unphosphorylated forms of phosphotyrosine-containing proteins and thus are substrates for tyrosine protein kinases. By two-dimensional gel analysis, we have found that the 46,000-dalton protein exists in two unphosphorylated forms of which the more acidic is a minor species. This latter form is phosphorylated, chiefly at serine, in both normal and transformed cells, generating a yet more acidic species. In transformed but not normal cells, the major form is phosphorylated at tyrosine and serine, yielding a fourth isoelectric variant. The 46,000-dalton unphosphorylated protein has been partially purified, and antiserum to it recognizes all four isoelectric variants of the protein. The 39,000-dalton protein has two unphosphorylated forms of which the more acidic is a minor species. The major form is phosphorylated at tyrosine and serine in transformed cells only. The 39,000-dalton unphosphorylated protein has been partially purified, and antiserum raised to it recognizes all three isoelectric variants. The 28,000-dalton protein has a single phosphorylated form which contains serine in normal cells, but both serine and tyrosine in transformed cells.