The effects of acute poisoning of copper ion on the serum protein and tissue protein patterns in the heart, liver, kidney, skeletal muscle, and ovary of catfish, Parasilurus asotus, were studied by cellulose acetate gel electrophoresis. There were 6 kinds of the serum protein fractions in the normal fish. When the fish were exposed to copper ion, the contents of prealbumin, albumin, and α₁-globulin were decreased and those of α₂-globulin, β-globulin, and Υ-globulin were increased. Tn the normal fish, there were 7 bands of the protein fractions in the heart. 5 in the liver, 3 in the kidney, 6 in the skeletal muscle, and 5 in the ovary. The different patterns of protein fractions exhibited the organ specificities. When the fish were exposed to copper ion, the numbers of tissue protein fractions were increased in the heart and kidney, and those were decreased in the skeletal muscle. However, the number of protein fractions of the liver and ovary was not changed. When the fish were exposed to copper ion, the relative mobilities of tissue protein fractions were broadened with the range from 0.17 to 0.94 in the heart, liver, kidney, and ovary. However, the relative mobilities of protein fractions in the skeletal muscle showed narrow range from 0.38 to 0.72. The molecular weights of protein fractions ranged from 9,500 to 230,000 in the 5 organs of the normal fish and varied from 3,500 to 170,000 in the experimental group.