Different bacterial strains were investigated for their benzylpenicillin destroying activity. The possible degrading products, benzylpenicilloic acid (BPA), 6-aminopenicillanic acid (6-APA) or penicic acid (PA) were screened by thin layer chromatography, using a new solvent system. Near all the strains tested, produce at different levels a @-lactamase as observed from the BPA production. Acylase was not produced by the strains investigated. Neither 6-APA nor PA were found, although small amounts of these products could not be detected by this qualitative method. BPA production from benzylpenicillin was measured quantitatively as a function of the enzymatic activity of the b-lactamase.These experiments were carried out on the washed bacterial cells, the cell-free growth medium and the cells treated with toluene. Most of t,he /?-lactamase activity was produced extracellularly by the bacteria; lower levels were found in the washed cell suspensions.The resistance of certain bacteria to benzylpenicillin has been shown to be associated with the destruction of the penicillin-G molecule by enzymatic activity of these organisms (ABRAHAM and CHAIN 1940, AYLIFFE 1965, HAMILTON- , HUBER et al. 1968, RICHMOND 1963, SMITH and HAMILTON-MILLER 1963. The experiments described hereafter were designed to investigate the activity of penicillin-G degrading enzymes among different bacterial strains and to ascertain the role of these enzymes in the resistance of bacteria t o penicillins. ABRAHAM and CHAIN (1940) first described the production of penicillinase (8-lactamase) by Escherichia coli and since then there have been many reports on the synthesis of this type of enzyme by other gram-negative species. Unlike the /?-lactanlases from gram-positive st,rains, these enzymes are not usually inducible (SMITH and HAMILTON-MILLER 1963). Among gram-negative bacteria, Klebsiella and Proteus species are described t o produce
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