In this study, the in vitro effects of some active cations on carbonic anhydrase (CA) isozymes were investigated. Bovine (BCA) and human (HCA-I and H CA-ll) isozymes were purified by affinity chromatography from bovine and human erythrocytes. In order to make the kinetic studies, esterase activities of CA isozymes were determined. By using AI{3+}, Mn{2+}, Sr{2+}, Hg{2+}, Ni{2+}, Ca{2+}, and Cd{2+}ions, K iconstants for CA isozymes were found by means of Linewear-Burk graphics. While A1{3+} showed competitive inhibition, the others displayed noncompetitive inhibition. In addition, the inhibitor concentrations halving the enzyme activity (I{50} values) were determined. The determined I{50} values for BCA, HCA-I and CA-ll were fit to the obtained Ki values.