SUMMARY: The properties (Michaelis constants, rates of inactivation at looo, temperature coefficients and pH/activity relationships) of the extracellular penicil- linase formed by a strain of Bacillus subtilis when growing in casein hydrolysate ( ='basal' enzyme) have been compared with those of the enzyme formed by the organism when growing in the same medium after addition of 1 unit benzyl- penicillin/ml. ( = 'induced' enzyme); no differences between the two preparations were detected. Similarly, no immunological or other differences were found between the basal and induced penicillinases of B. cereus, strain 569. However, the B. subtilis penicillinase was significantly different from the B. cereus penicillinase in the value of its Michaelis constant and the shape of its palactivity curve; it was also quite distinct immunologically. In most cases of microbial enzyme adaptation (' induced enzyme synthesis ') the effect of the inducer is to cause a specific increase in activity of an enzyme which is, nevertheless, present to some extent in untreated cells. This spon- taneously produced enzyme has been referred to as 'constitutive' or 'basal'; and, according to the enzyme and organisms concerned, may form a consider- able or very minute fraction of that formed under influence of the inducer. In some instances no basal or constitutive enzyme activity has been detect- able. With the B-galactosidase of Escherichia coli ML studied by Monod and his colleagues, Cohen-Bazire & Jolit (1953) showed that the trace of basal activity found in cultures grown in maltose could be explained by the known presence of a small proportion of mutant cells which were capable of forming large quantities of the enzyme constitutively. How far the property of pro- ducing an enzyme spontaneously (i.e. without added inducer) is distributed equally amongst a cell population, may in most cases be largely unknown, but in general, the process of enzyme adaptation must be regarded as an increase in the rate of production of an enzyme which is formed to some extent in the absence of the inducer, and not a formation de nouo. The question is thus posed: Is constitutive (basal) enzyme composed of the same molecular species as induced enzyme, and if so, to what extent do the processes leading to their formation follow the same metabolic pathway? In the work reported here an attempt has been made to obtain an answer to the first part of the question by a comparative study of the biochemical properties of penicillinases from a strain (749) of Bacillus subtilis and from a strain (569) of B. cereus formed (a) spontaneously, and (b) under the influence of benzylpenicillin. This problem has not been studied systematically before. 32 G. Microb. XI