AbstractA procedure is given for the preparation and isolation of (‐)‐allantoin (5‐ureidohydantoin). A grown culture ofPseudomonas aeruginosawas incubated with (±)‐allantoin at pH 6. (+)‐Allantoin was degraded by the bacteria and (‐)‐allantoin was removed from the medium with the aid of activated carbon.Several properties of (‐)‐allantoin were determined: solubility and molar rotation in various solvents, infrared spectrum, decomposition point and dependence of the molar rotation on pH. The properties were compared with those of (±)‐allantoin,R‐ andS‐hydantoin‐5‐acetic acid.From circular dichroism measurements of (‐)‐allantoin,S‐hydantoin‐5‐acetic acid andR‐carbamoylalanine it was c