A specific novel molecular form of ceruloplasmin (CP) was detected in the sera of Wilson's disease patients and their closest relatives using two-dimensional cross-immunoelectrophoresis. This protein shares some antigenic properties with normal CP but is not completely identical to the latter. Besides, anomalous CP has no oxidase activity of normal CP and differs from it in electrophoretic mobility in agarose and polyacrylamide gels. Anomalous CP was purified to homogeneity and monospecific antibodies to this protein were obtained. The quantitative analysis showed that the ratios of normal CP to anomalous CP in homo- and heterozygous carriers of the "Wilsonian" gene are reproducibly different and can be used as a diagnostic test allowing the differentiation between these groups.