Cell surface polypeptides serve as substrates for a casein kinase-like ecto-protein kinase activity which is demonstrable under stringent criteria with intact cells using micromolar levels of extracellular [gamma-32P]ATP. Two major 32P-labeled proteins, designated as pp100 and pp120 after their apparent molecular masses on SDS-PAGE under reducing and nonreducing conditions, have repeatedly appeared in the phosphoprotein spectra of different cell types. We have chosen HeLa cells as a source for the biochemical characterization and isolation of pp100 and pp120. Phosphorylation of pp100 and pp120 occurs in their extracellular domains at seryl residues of amino acid side chains. Several criteria deduced from the heparin sensitivity of the ecto-protein kinase and its substrate-induced shedding into the cell supernatant indicated that surface phosphorylation is a function of the ecto-protein kinase. The radioactive phosphorylation of pp100 and pp120 which coincides with their biotinylation on 2D-blots can be reversed by mild trypsination of intact cells. Purification and enrichment of pp100 and pp120 were achieved on the basis of radioactivity detection on and isolation from 1D- and 2D-gels. Amino acid sequence analysis performed on tryptic digests of purified ecto-phosphoproteins in most cases showed significant consensus sequences between pp100 and the nuclear RNA-binding protein nucleolin while pp120 sequences proved to be related to hnRNP U, a nucleoplasmic pre-mRNA-binding protein. Immunochemical analysis using anti-nucleolin and anti-hnRNP U antibodies combined with comparative phosphorylation and characterization of the ecto-proteins with authentic nucleolin and hnRNP U further established the close relationship, suggesting surface membrane versions of the nuclear proteins.