1. A new automated micro-iodometric method is described for screening compounds for inhibitory action against β-lactamase enzymes. 2. Over 1000 semi-synthetic penicillins were tested for inhibitory activity against the β-lactamase of Escherichia coli B11 and 18 showed a fractional inhibition similar to or higher than that of methicillin. 3. The best inhibitors were alkoxy- and halogen-substituted phenyl-, naphthyl- or quinolyl-penicillins. 2-Isopropoxy-1-naphthylpenicillin (BRL 1437) was clearly the best and had a Ki value about 1% of that of methicillin. 4. The inhibition of the β-lactamase of E. coli B11 by BRL 1437 was shown to be reversible and competitive. The Ki was 0.004μm and Ki/Km with ampicillin and p-hydroxyampicillin (BRL 2333) was about 0.0001. The Km and Vmax. values were determined for the β-lactamases of E. coli B11 and Klebsiella aerogenes A against a variety of penicillins. Cell-bound and solubilized enzymes gave similar Ki and Km values. 5. BRL 1437 was superior to cloxacillin and methicillin for inhibition of the β-lactamase of live, fully grown cultures of several strains of E. coli and K. aerogenes. Of a group of inhibitors BRL 1437 was the most stable to the β-lactamase of E. coli B11.

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