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Oral contraceptives (OC) can induce in some women, scrum immune complexes precipitated in 25% saturated (NH4)2SO4, especially in the case of a thrombotic complication. In this work, the presence of antiethinyloestradiol (anti-EE) antibodies in the complexes was investigated. Binding of ethinyloestradiol-³H (EE-³H), either by (NH4)2SO4, precipitate directly, or in equilibrium dialysis experiments with the anti-EE gamma-globulin isolated through a purification-activation method combining dilution chromatography and affinity chromatography, was measured. The purified proteins were identified by immunoprecipitation and isoelectrophoresis. Twelve sera from OC users and three from women who had never used them were tested. Eleven of the twelve users had serum immune complexes. Normal human gamma-globulin pools and gamma-globulins prepared from a rabbit immunized with EE, were used as negative and positive controls. Results demonstrated that serum complexes precipitated in 25% saturated ammonium sulphate contained EE-³H binding gamma-globulins. For seven out of eleven cases, the association constant Ka was calculated and varied from 4 × 105 M-1 to 2.6 × 107 M-1, and the valence from 1.4 to 2, a result consistent with an IgG antibody activity. These anti-EE antibodies were found in the serum complexes from women on OC with or without thrombosis. However their amount seemed to be greater in the cases with thrombosis. In the serum of women who had never used the pill, in the only OC user who had no serum complexes in 25% saturated ammonium sulphate, and in the pool of normal human gammaglobulins, no anti-EE activity was detected. [ABSTRACT FROM AUTHOR]