Protein S is a plasma protein that serves as a cofactor for the anticoagulant effects of activated protein C. Congenital protein S deficiency is often associated with thromboembolic disease. During pregnancy a decrease in the functional and antigenic levels of protein S occurs; this change in protein S status may contribute to the thromboembolic complications that sometimes occur during pregnancy. In certain patients, oral contraceptive use has also been associated with thrombotic complications. In this study, protein S status was determined in 21 women taking oral contraceptives and compared with that of 21 women not taking oral contraceptives and that of 21 men. The results show that women taking oral contraceptives have significantly lower total protein S (24.3 +/- 3.6 micrograms/mL; mean +/- SD) than women not taking oral contraceptives (28.6 +/- 3.9 micrograms/mL) (P less than .005). Men had significantly higher protein S levels (30.9 +/- 3.9 micrograms/mL, P less than .01) than age-matched women not taking oral contraceptives. In plasma, an equilibrium exists between free (functionally active) protein S and protein S complexed to C4b-binding protein, which is functionally inactive. The mean levels of C4b-binding protein were essentially the same among the three groups, but the levels of free protein S were significantly different and reflected different total protein S antigen levels. Additionally, we found that inflammation significantly elevated C4b-binding protein levels and could result in a further significant decrease in free protein S levels. These data indicate that plasma protein S levels are significantly affected by hormonal status and inflammation.