Primary cell walls, free from cytoplasmic contamination were prepared from corn (Zea mays L.) roots and potato (Solanum tuberosum) tubers. After EDTA treatment, the bound acid phosphatase activities were measured in the presence of various multivalent cations. Under the conditions of minimized Donnan effect and at pH 4.2, the bound enzyme activity of potato tuber cell walls (PCW) was stimulated by Cu(2+), Mg(2+), Zn(2+), and Mn(2+); unaffected by Ba(2+), Cd(2+), and Pb(2+); and inhibited by Al(3+). The bound acid phosphatase of PCW was stimulated by a low concentration but inhibited by a higher concentration of Hg(2+). On the other hand, in the case of corn root cell walls (CCW), only inhibition of the bound acid phosphatase by Al(3+) and Hg(2+) was observed. Kinetic analyses revealed that PCW acid phosphatase exhibited a negative cooperativity under all employed experimental conditions except in the presence of Mg(2+). In contrast, CCW acid phosphatase showed no cooperative behavior. The presence of Ca(2+) significantly reduced the effects of Hg(2+) or Al(3+), but not Mg(2+), to the bound cell wall acid phosphatases. The salt solubilized (free) acid phosphatases from both PCW and CCW were not affected by the presence of tested cations except for Hg(2+) or Al(3+) which caused a Ca(2+)-insensitive inhibition of the enzymes. The induced stimulation or inhibition of bound acid phosphatases was quantitatively related to cation binding in the cell wall structure.