Protein S, is a natural anticoagulant protein which serves as a cofactor for activated protein C. During pregnancy and in the postpartum period, functional protein S levels are significantly reduced (38% +/- 17.3%, mean +/- 1 SD) when compared to nonpregnant females (97% +/- 31.6%) (P less than 0.001). In plasma an equilibrium exists between functionally active free protein S and protein S complexed with C4b-binding protein, which is functionally inactive. As a result of this equilibrium either a decreased level of total protein S antigen or an elevation of C4b-binding protein could lead to reduced protein S activity. C4b-binding protein levels measured by enzyme- linked immunoassay are not significantly different in pregnant women versus nonpregnant controls (103.5% +/- 21.2% v 100% +/- 16.9%). However, during pregnancy and in the postpartum period, total protein S levels are reduced (68% +/- 10.7%) compared to nonpregnant controls (100% +/- 17.0%). This difference is significant at P less than 0.001. These data demonstrated that the reduction in protein S activity observed during pregnancy is a result of reduced total protein S antigen.