The activity of cytochrome c oxidase was highest in spleen and was decreased in the order of newborn baby, lung, fetus, heart, kidney, brain, liver, and muscle. The electrophoretic mobility of the enzymes from spleen, lung and newborn baby was fast: the one from heart, kidney, muscle and liver was middle: the other from fetus and brain was slow. The isoelectric point (pI) of the enzyme of lung, heart, kidney, brain, spleen, and both fetus and newborn baby was 6.5, 5.7, 5.9, 7.6, 6.1, and 5.8, respectively. The liver enzyme was appeared to have two pIs of 5.0 and 7.3