The inducible beta-lactamase from Pseudomonas aeruginosa NCTC 8203 and the constitutive beta-lactamase from strain 1822 S/H have been isolated and compared. These two enzymes are apparently periplasmic since they are released by freezing and thawing. They resemble each other closely in their molecular weights, amino acid composition, isoelectric points and electrophoretic mobility as well as in their catalytic properties, and they may be identical. Neither enzyme contains a free thiol group.