Peptide acceptor properties of phenylalanine and glycine esters of 9'-(2',3'-dihydroxypropyl-1')-adenine and 1-(2',3'-dihydroxypropyl-1')-4-thiouracyl were investigated. All these esters appeared to be powerful inhibitors of polyphenylalanine synthesis in E. coli MRE-600 ribosomes charged with poly U. Like puromycin, esters of adenine derivatives accepted the AcPhe residue from Ac-[(14)C] Phe-tRNA in a ribosomal system charged with poly U. However, peptidyl esters of 9-(2',3'-dihydroxypropyl-1')-adenine remained bound with ribosomes. The structure of the peptide esters synthesized was ascertained after dissociation of ribosomes into subparticles by direct comparison with the synthetic specimens.