Three carbenicillin-hydrolyzing penicillinases were found in Proteus mirabilis strains, N-3, N-29, and GN79. The former two strains were isolated in 1978, but strain GN79 was one of our stock cultures isolated in 1965. These penicilIinases closely resembled each other, and the PSE-l and PSE-4 enzymes produced by Pseudomonas aeruginosa, in their substrate profiles and kinetic properties for hydrolyzing various tJ-lactams. However, differences were found in their molecular weights and isoelectric points which ranged from 22,000 to 27,000 and from 6.0 to 6.9, respectively. The antiserum against the purified penicillinase of N-29 cross-reacted with the enzyme of N-3 and inhibited its activity by more than 80%. The antiserum also reacted with the PSE-l and PSE-4 enzymes. The antiserum did not react with the penicillinase from strain GN79 and the PSE-2 and PSE-3 enzymes of P. aeruginosa, Enzyme production in N-3 and N-29 was mediated by R plasmids.Many .B-lactamases [EC 3.5.2.6] mediated by chromosomal gene(s) or those carried on plasmids have been extensively studied by many workers (4,23). These enzymes have been classified according to such properties as substrate specificity, interaction with inhibitors and inactivators, isoelectric point, molecular weight, and immunochemical properties. There are two major groups of .B-lactamases, i.e. the penicillinase type and cephalosporinase type. In gram-negative rod bacteria, most of the penicillinases are genetically governed by R plasmids. Among these, penicillinases mediated by R plasmids occurring predominantly in Pseudomonas aeruginosa are known to effectively hydrolyze carbenicillin (11). The enzymes were termed PSE (Pseudomonas-specific enzymes), and were divided into four subgroups, i.e., PSE-l, PSE-2, PSE-3, and PSE-4, by Hedges and Matthew (10). However, recently, Medeiros et al (16) demonstrated that PSE-l enzymes mediated by R plasmids were spread among enteric bacteria such as Escherichia coli, Salmonella enteritidis, and Shigella sonnei, and Katsu et al (12) have reported PSE-l enzymes mediated by R plasmids in Proteus mirabilis.We also found that the penicillinase of P. mirabilis GN79 had the same characteristics as carbenicillin-hydrolyzing penicillinase (carbenicillinase). This strain was isolated in 1965 (26) and the penicillinase was considered to be a species-specific 995

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