The synthetic hormones contained in contraceptive pills were shown to induce antiethinylestradiol (EE) antibodies in some women. These antibodies can be detected by the presence of circulating immune complexes (CIC) which are precipitated from serum in 25 p. cent saturated ammonium sulphate. A method of analysis of the anti-EE antibodies is described, in which binding of tritiated EE is measured with and without addition of unlabeled EE in excess. This method allows to identify specific reversible binding on saturable sites. Because of the large excess of non specific sites, the whole serum was enriched in antibodies before the binding measurement, either by previous separation of CIC, or by an affinity chromatography on a EE column. Results of this method confirmed the presence in a number of women on oral contraceptives, of immunoglobulins which were able to bind EE reversibly. The antibodies were found in CIC. in the absence of CIC, they were occasionally found in serum after an affinity chromatography on a EE column. This method is felt to allow a more accurate detection of women immunoreactive to the pill.