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Apolipoprotein A-I exhibits a polymorphism that can be easily investigated in native serum by a simple method involving incubation of serum in the presence of decylsulfate and beta-mercaptoethanol and subsequent isoelectric focusing. From six to eight proteins can be separated in a pH gradient from 4 to 6 and thus patients with apolipoprotein A-I variants can be distinguished from normal persons. This method also permits monitoring for polymorphic forms of apoA-II and apoA-IV as well as detection of C apolipoproteins. To verify the identity of the different apolipoproteins, a two-dimensional electrophoresis technique was applied, with an SDS system for the second dimension. In addition, monospecific antibodies for apolipoproteins A-I, A-II, and A-IV were used for the immunological identification. The method described here led to the discovery of three different familial apolipoprotein A-I variants.