The persistence and stability of proteins, including enzymes, in soils are generally attributed to their association with clays and humus. The adsorption of enzymes by clay minerals results in significant changes in enzyme properties, such as pH optimum, stability, activity, and kinetics. The apparent pH optimum of clay-adsorbed enzymes is generally displaced one or two pH units to more alkaline values. The activities of clay-adsorbed enzymes are often lower than those of free enzymes in homogeneous solution. Adsorption of enzymes by clays may also affect the kinetics of enzyme-catalyzed reactions. The concept of enzyme cofactors or of chemicals that act as cofactors, performing biochemical functions in concert with inorganic matrices would seem to merit further interest. The effects exerted by organic surfaces may be important in determining the activities and stabilities of enzymes associated with organic matter and clay-organic matter complexes in soils.