The influence of Cu, Zn, and Cd on the activity and kinetics of acid phosphatase immobilized by two soil clays, kaolin or goethite was investigated. The ability of Cu to inhibit the enzyme activity was higher than that of Zn in all the enzyme complexes examined. The ability of Cd was negligible. The inhibitory effects of Cu and Zn on the two soil clay- and kaolin-enzyme complexes were much stronger than those on the goethite-enzyme complex. The Vmax and Km values of the enzyme complexes indicated that both Cu and Zn decreased the maximum reaction velocity of the enzymes, but increased the affinity of the enzymes for the substrate. The degree of the decrease and increase was higher in the Cu systems than in the Zn systems.