SUMMARY: Production of βf-lactamase by nine strains of the genus Enterobacter (eight Enterobacter cloacae, one E. aerogenes) was studied to determine its in-ducibility. Induction was observed with benzylpenicillin (500 μg./ml.) in all except one strain. Cultures were examined to locate the enzyme; it was found that in exponential growth the enzyme was cell-bound, and in stationary phase cultures much of it was in the culture medium. Maximum enzyme activity was only demonstrated after cell-breakage. Substrate profiles of crude enzyme preparations were examined and it was observed that the enzymes were 20–80 times more active against cephalosporins than against benzylpenicillin. Evidence is presented which suggests that one strain of E. cloacae produced two β-lactamases, an inducible cephalosporinase and a constitutive peni-cillinase.