A streptavidin-beta-galactosidase fusion protein was expressed in Escherichia coli and bioselectively adsorbed from crude cell lysates to biotin covalently immobilized on controlled pore glass. Michaelis constants for the immobilized enzyme with o-nitrophenyl-beta-D-galactopyranoside and lactose were 0.28 and 0.4 mM, respectively. Very similar values were obtained with a commercial preparation of soluble enzyme, indicating that neither folding of the fusion protein nor interaction of the streptavidin domain,vith immobilized biotin altered the structure of the substrate binding site. As compared to soluble enzyme, the apparent optimum pH for activity was shifted 0.5 units to the acid region and the optimum temperature was 5 degrees C lower. Similar bioreactor activities were regenerated five times by desorption of the fusion enzyme protein with 6 M guanidinium chloride followed by readsorption from cell lysates.