The effects of montmorillonite (Osage, Wyoming) with different interlayers on the adsorption, activities, and kinetic properties of acid phosphatase were examined. Hy-droxy polymers of aluminum or chromium (200 cmol kg-1 clay) were intercalated in montmorillonite in the absence or presence of citric acid (citrate/Al or Cr=0.1 and 0.5). The activities of acid phosphatase in the presence of different amounts of clay at three pH values (5.0, 5.5, and 6.0) were de-termined, and the Michaelis-Menten kinetic parameters were calculated. The amount of enzyme adsorbed by interlayered montmoril-lonite was considerably lower than that by pure montmorillonite and increased as the citrate/Al or Cr ratio increased. The type of interlayer (A1 or Cr) influenced the ease with which enzymes could be removed by washing. Complete adsorption of the enzyme led to the formation of clay-enzyme com-plexes, which had lower activities than the free enzyme. The order of activity of en-zymes following adsorption was Cr-montmo-rillonite>Al-montmorillonite>montmoril-lonite. The effect of citrate on enzyme activity in the clay-enzyme complexes was significant only at the higher ratio. Factors that caused a reduction in enzyme activity were adsorption by clay, amount of clay, and increase in pH. These effects were modified by the type of interlaycrs (AI or Cr), the ci-trateYAI or Cr molar ratio, and the strength of adsorption. In the presence of clay, the Michaelis-Menten affinity constant (K.,), in-creased, whereas the luaxinlnnl eazyme reaction velocity (Vmax), decreased compared with the free enzyme. The change of the enzyme kinetics by A1- and Cr-interlayered montmorillonite can be described as partial noncompetitive inhibition.