Scrum levels of α2-macroglobulin, IgG, albumin and transferrin were estimated immunochemically in 45 pregnant women and 45 matched nonpregnant controls. There was a significant rise in α-macrogiobulin and transferrin but a decrease in IgG and albumin. Cryofiforinogenemia was detected in 389o of the samples from pregnant women, compared with 7% of the controls. The increase in u_-macroglobulin, possibly due to hormonal changes, may be one of the significant factors contributing to the increased incidence of thrombocmbolism during pregnancy. The frequency with which cryofibrinogenemia is detected in pregnant women points to the existence of subclinical in Ira vascular coagulation.