Beta-DL-Thiaproline (thiazolidine 2-carboxylic acid) is a good substrate for hog kidney D-aminoacid oxidase. Unlike other known substrates, beta-thiaproline is better oxidized at neutral than at alkaline pH. At neutral pH beta-thiaproline is a better substrate than D-proline. Beta-DL-thiaproline is fully oxidized to delta 2 thiazoline 2-carboxylic acid, which in acidic medium is hydrolyzed to N-oxalylcysteamine. These results may support the suggestion that beta-thiaproline, arising in vivo from cysteamine and glyoxylate, can be a possible physiological substrate for D-aminoacid oxidase.