5-Aminolevulinic acid dehydratase (ALA-D) in blood of carp Cyprinus carpio can be used as an indicator of water lead (Pb) pollution, because ALA-D activity is inhibited by lead contamination. The present study was performed in order to examine the type of inhibition of ALA-D activity in carp blood caused by Pb and cadmium (Cd), in order to explain the reason why the blood ALA-D activity of Pb-contaminated carp is particularly depressed, and in order to ascertain the effects of the pH of the reaction mixture and the reaction temperature on the ALA-D-catalyzed reaction of carp blood. The results were summarized as follows. 1) In the case of 0.1M phosphate buffer, a graph of the pH of the reaction mixture versus ALA-D activity showed a broad peak between 5.5 and 6.4 with the maximum activity at pH 6.0. 2) At temperatures of up to 60℃, ALA-D activity increased rapidly with elevating reaction temperatures, however, this activity decreased sharply at temperatures greater than 70℃. 3) Under in vitro ALA-D-catalyzed reaction in reaction mixture containing Pb and Cd, whilist comparing the blood ALA-D of Pb-contaminated fish with that of control fish, increasing substrate concentrations were not found to alter the Km value and shifted the Vmax (maximum velocity). Therefore, it was concluded that the type of in hibition of blood ALA-D activity caused by Pb and Cd is non competitive inhibition. Since the activity values of Cd-contaminated fish were similar to those of the control fish, the exposure of carp to water Cd did not inhibit blood ALA-D activity. 4) Pb of remarkably high levels was accumulated in the blood corpuscles of Pb-contaminated fish. However, the Pb level of the blood plasma was similar to that of the control fish. On the other hand, in comparison with that of the control fish, Cd levels both in the corpuscles and in the plasma of Cd-contaminated fish did increase, however unlike the Pb levels in the blood corpuscles, they did not increase remarkably. 5) An inhibition of blood ALA-D activity was observed in the Pb-contaminated carp, but was not observed in the Cd-contaminated carp. The reason for this can be explained by the following. Pb taken up through the gills became accumulated at a remarkably high level within the blood corpuscles. However, unlike Pb, the Cd taken up did not become accumulated at high levels within the corpuscles. Blood ALA-D is an enzyme which is present in erythrocytes, and erythrocyte ALA-D combines with Pb within the blood corpuscles. The inhibitory degrees of blood ALA-D activity caused by Pb, depend upon the amounts of Pb which become accumulated within the blood corpuscles.