Article describing the preparation and chemical characterization of low molecular weight conjugates that were used as models for an acetal linkage between ADP-ribose and the hydroxyl group of a protein acceptor such as serine, threonine, tyrosine, hydroxyproline, or hydroxylysine residues. The amount of modification was approximately 16 pmol of ADP-ribose per mg of total protein, and proteins modified by acid-labile linkages were detected in all subcellular fractions examined, suggesting that the scope of this modification in vivo is broad.