E. coli contains two proteins which are covalently linked to nucleoside monophosphates (AMP and UMP) through a tyrosine phosphodiester bond. Recently, it has been shown that mammalian cells contain phosphorylated tyrosine residues in proteins, and in one case a viral protein covalently linked to polynucleotide through a tyrosyl phosphodiester bond. Present work involves the detection and quantitation of proteins containing either of these modifications by special protein hydrolysis techniques for tyrosine phosphate and specific antibody recognition for adenylylated and uridylylated tyrosine.