Alkaline phosphatase from bovine intestinal mucous membrane was immobilized by sorption on Na-sepiolite in order to obtain an enzyme-clay complex for soil application. Optimum conditions for immobilization were: phosphate buffer 0.1 M at pH 7, 2 h of magnetic stirring at 4 degrees C, 0.25 mg ml(-1) for the concentration of enzyme and an enzyme/clay ratio of IO ml g(-1). The adsorption isotherm obeyed the Langmuir equation (r(2) 0.996 at P<0.001), reaching a maximum amount of enzyme retention. Apparent K-m values were 18.32 mM for the immobilized enzyme and 2.36 mM for the enzyme in aqueous solution; optimum pH (10.5) did not change as a consequence of immobilization; a first-order kinetic was obtained for incubation times of less than I h for both enzymes and the immobilized enzyme showed a lower incubation temperature optimum. The immobilized phosphatase showed less stability than the soluble enzyme in terms of storage at 30 degrees C, thermal stability and resistance to proteolytic attack.