1. A procedure was devised which is suitable for the isolation of β-lactamase I and β-lactamase II from Bacillus cereus 569/H/9 on a large scale. After adsorption on to Celite both enzymes were eluted in good yield and separated by chromatography on Sephadex CM-50. 2. β-Lactamase I was separated into three main components by isoelectric focusing and into two components by chromatography. 3. The Zn2+-requiring β-lactamase II obtained by this procedure had a lower molecular weight (22000) than β-lactamase I (28000) and also differed from the latter in containing one cysteine residue. 4. The β-lactamase II contained no carbohydrate, but showed the thermostability of the enzyme isolated earlier as a protein–carbohydrate complex. 5. Amino acid analyses and tryptic-digest ‘maps’ indicate that some degree of homology between β-lactamase I and β-lactamase II is possible, but that β-lactamase I is not composed of the entire sequence of β-lactamase II together with an additional peptide fragment. 6. A 6-methylpenicillin and a 7-methylcephalosporin showed much lower affinities for both enzymes than did penicillins and cephalosporins themselves.
