Crossed immunoelectrophoresis of human serum revealed two heterogeneity types of ceruloplasmin with different electrophoretic migration. The two types both consisted of peptides with Mr 150 000, 100 000 and 45 000, which were interpreted as native ceruloplasmin and two hydrolytic fragments. The two types were different in copper content, and one type could reversibly be changed into the other. The glycan microheterogeneity of ceruloplasmin was analyzed by crossed affinommunoelectrophoresis with free Lens culinaris agglutinin (LCA) and wheat germ agglutinin (WGA). A third of the ceruloplasmin molecules, both high and low copper type, bound to LCA and two thirds to WGA. The heterogeneity and the microheterogeneity of ceruloplasmin in two groups of patient sera were compared to sera from healthy individuals. The ceruloplasmin type with respect to copper content was a much better factor than either glycan microheterogeneity or total serum concentration in discriminating between the three groups.