ABSTRACT:Circulating immune complexes were detected in women on oral contraceptives (OC) by a simple antigen nonspecific method using precipitation of serum in 25% saturated ammonium sulfate (CIC‐AS). A significant correlation was found between the presence of CIC‐AS and the OC vascular risk.A radioimmunoassay with tritiated ethinylestradiol indicated that CIC‐AS contained antiethinylestradiol antibodies (anti‐EE Ab) in a number of OC users, but indicated also that 1) anti‐EE Ab may be found in cases with no detectable CIC‐AS, 2) CIC‐AS containing no anti‐EE Ab are found in nonusers, and 3) even in OC users the CIC‐AS may contain antibodies to other ligands than EE.The study demonstrated also that, in OC users with a vascular complication, anti‐EE Ab were more frequently detected (78% of cases) than CIC‐AS (60%). Moreover, among OC users with CIC‐AS, anti‐EE Ab were found in 95% of women with a vascular complication and only 37% of current healthy users.The detection of anti‐EE Ab appears to be more predictive, with regard to the vascular risk of OC, t