Two different fractions with $delta$-aminolevulinate dehydratase activity designated isozyme I and II tentatively were isolated from liver homogenate of ICR inbred mice by slightly modifying the procedure of Doyle and Schimke, and further their physicochemical properties were characterized as follows. 1. Optimal activities of the isozyme I and II appeared to be pH 6.5 in phosphate buffer. However, catalytic activities of the both isozymes were shown to be more active in Tris-HCl buffer at pH 7.5 than in the phosphate buffer at the same pH. 2. Both of the isozymes were found to be relatively heat stable, of which stability of the isozyme I was more resistant comparable to isozyme II. 3. Apparent Michaelis-Menten Constants of the both isozyme I and II in the present experiment were $2.1{ imes}10^-4M$ and $5.5{ imes}10^-4M$, respectively. 4. Inhibition of the enzyme activity by heavy metals such as $Pb^{++}$ and $Hg^{++}$ in vitro was more debiliated in isozyme II than in isozyme I. 5. It was found that isozyme I could not be isolated by the same procedure as was done in the normal group from lead nitrate-treated group. In addition, the enzymatic activity of isozyme II was also decreased remarkably by the same treatment. The animals used have been treated by injecting intraperitoneally 1% lead nitrate solution with 1.3 ml/100 gm of body weight to each mouse 3 weeks prior to performing the experiment. 6. Both the isozyme I and II showed the different electrophoretic mobilities on immunoelectrophoresis. Rabbit anti-sera to isozyme II have been applied.