In model experiments the influence of calcium-homoionic clay minerals (montmorillonite, illite, and kaolinite) on the activity, Km, and Vmax, values of alkaline phosphatase was studied. Increasing amounts (50,100, and 150 milligrams, respectively) of each clay at different substrate (= p-nitrophenol phosphate) concentrations decreased the enzyme activity. The inhibiting effect of 150 milligrams of illite (64 percent) was considerably higher than those recorded with the same amount of montmorillonite (37 percent) or kaolinite (22 percent). In the presence of montmorillonite or kaolinite, the Michaelis constant Km became enhanced (7.89 and 6.05 X 10-3 M p-nitrophenol (phosphate, respectively) compared to the soluble state (Km, = 4.26 X 10-3 M p-nitrophenol phosphate). The values of the maximum enzyme reaction velocity Vmax (= 2553 micrograms p-nitrophenol x 10 ml-1 x h-1 in the soluble state) was accelerated in the presence of montmorillonite (Vmax = 2778 micrograms p-nitrophenol x 10 ml-1 x h-1), but decreased with kaolinite (Vmax = 2469 micrograms p-nitrophenol x 10 ml-1 x h-1). In the presence of illite, however, both Km (= 3.92 X-3 Mp-nitrophenol phosphate) as well as Vmax (= 980 microgramsp-nitrophenol X 10 ml-1 x h-1) were diminished clearly. This increased enzyme-substrate formation in the presence of illite in combi- nation with a lowered hydrolysis of the sorbed enzyme-substrate complex, is explained by a modification of the alkaline-phosphatase configuration as a consequence of sorption. Such a modification in the alkaline-phosphatase configuration could have facilitated the enzyme-substrate complex formation but may have increased its stability against subsequent hydrolysis.