Penicillin acylase can be used for the synthesis of beta-lactamic antibiotics which have a wide range of applications, however, the utility of this synthetic route is often limited by the necessity of using organic solvents which have both an inhibitory effect on the enzyme activity and a deleterious effect on the enzyme stability. The first limitation can be overcome by careful selection of the organic solvent so as to ensure that high synthetic yields ave obtained without excessively reducing the activity of the enzyme derivative, and the second by using enzymes previously stabilized In this paper, the synthetic and hydrolytic behavior of stabilized derivatives of penicillin acylase has been rested in various water-cosolvent mixtures. The overall performance of the enzyme in the synthetic mode is a combination of two effects : (i) the inhibitory effect of the solvent on the enzyme ann (il) the desirable increase in the nonionized form of the substrate in the presence of solvent. The ratio of the enzyme activity in its synthetic mode to its hydrolytic mode was found to be a useful guide Sor the selection of a suitable solvent.