The preparation and properties of Escherichia coli beta-galactosidase conjugates based on a recently developed immobilization method are presented. Enzyme immobilization is performed by reaction of its native thiol groups with agarose-bound thiolsulfonates under mild conditions. Depending on the amount of enzyme applied, 8-114 mg of protein per gram of dried gel derivative was immobilized on the thiolsulfonate agarose. With low protein loadings, the immobilization yield reached 100%. The thiolsulfonate-agarose gels exhibited high selectivity toward active enzyme. Because of this, the specific activity of the immobilized beta-galactosidase was up to 50% higher than that of the applied, native enzyme, The method also provides the possibility to design the microenvironment of the immobilized enzyme by blocking residual thiolsulfonate groups with different reagents. The low-bad derivatives blocked with glutathione, as well as the high-load derivatives without blocking, had better thermal stability than the soluble enzyme, and also showed excellent long-term stability at low temperatures. Thus, no decrease in enzymic activity was observed after storage of the derivatives for 18 months at +4 degrees C as suspensions in 0.1 M potassium phosphate, pH 7.5.