THE presence of the enzyme penicillinase in extracts of B. coli and other micro-organisms was first described by Abraham and Chain1. Later, a unit of penicillinase potency was proposed by McQuarrie, Liebmann, Kluener and Venosa2. Since that time several groups of investigators have studied this enzyme3–11 and defined their own relative units of activity. This was justified by the fact that no pure crystalline product suitable as a reference standard was available. The definitions of these units are all based on arbitrary experimental conditions of inactivation and techniques of penicillin assay, and thus do not permit interconversion. Moreover, the majority of the techniques used are based on penicillin inactivation at the lower ranges of concentrations at which the rate of destruction is a function of both the penicillinase concentration and the penicillin concentration, while the latter is decreasing during the experiment.