Sera from patients with proven cases of syphilis were tested for the presence of antibodies to structurally important phospholipids by using qualitative and quantitative assays. All 47 sera examined qualitatively contained antibodies to cardiolipin, phosphatidic acid, and phosphatidylserine, but not antibodies to other selected phospholipids. Such reactivity was not found in normal (Red Cross) sera. Although the degree of antibody binding to phospholipids varied in individual sera, reactivity was almost always greater with cardiolipin than with phosphatidic acid or phosphatidylserine. Binding saturability was found in sera as the cardiolipin concentration was increased over a constant area of nitrocellulose paper. Anti-cardiolipin binding measured by the protein A method gave results similar to the results measured by using anti-immunoglobulin G, which supports the conclusion that binding was to the Fab portion of the immunoglobulin molecule. When measured as a function of serum concentration and plotted in double-reciprocal fashion, the anti-cardiolipin binding data for two syphilitic sera had similar Kd values but different Bmax values. Stoichiometric calculations indicated that approximately 11,000 to 16,000 mol of cardiolipin appeared to be bound per mole of labeled second antibody. These observations may mean that the anti-cardiolipin antibody does not recognize the individual cardiolipin molecule as the antigenic site but recognizes some structural form of the phospholipid or that steric hindrance related to the interaction of the phospholipid with nitrocellulose paper prevented the bulk of cardiolipin molecules from reaching. The structural specificity of the antibodies identified excludes the possibility that these antibodies are directed against the phosphodiester linkage. These findings should give impetus to future study of a potential pathogenic or marker role for these antibodies in syphilis and in other syndromes in which membrane damage may be a primary event.