A method for optimization of the cell disruption process in order to improve the release selectivity and efficiency of a target intracellular enzyme was investigated. The cell concentration as measured by optical density was found to be an effective measure for the determination of the degree of cell disruption. Based on a kinetic analysis of the cell disruption process, generalized operational parameters were found to apply depending on the specific energy supplied. Generalized values for the disruption rate constant K-C and the release rate constant for each enzyme K-i (i=1, 2, ...) were well correlated with the cell wall strength as a function of the disruptive stress and with the intracellular location of the enzyme, respectively. Determined values of the release selectivity, K-i/K-T, were found to be an indication of the intracellular location of the enzyme. The disruption medium was modified by the addition of polyethylene glycol 1540 to enhance the release selectivity of the target beta-galactosidase from Escherichia coli ML308. The addition of polyethylene glycol from 0 to 5% was found to increase the release selectivity of beta-galactosidase from a value of 1.2 to 3.0. The disruption condition (specific energy input) was optimized to give both the higher yield and selectivity.