Three low-dose oral contraceptives Trinordiol, Gynatrol, and Marvelon, containing ethinylestradiol (EE) in combination with triphasic levonorgestrel (LNg), monophasic levonorgestrel, and monophasic desogestrel (DGS), respectively, were given to 65 healthy women, n = 21-22 in each group. Blood levels of antithrombin III (AT III), alpha 2-macroglobulin (alpha 2M) alpha 1-antitrypsin (alpha 1at), complement factors (factor B, C3, C4), pregnancy zone protein (PZP), corticosteroid binding globulin (CBG), sex hormone binding globulin (SHBG) and albumin were measured before treatment and during the first and third treatment cycles. AT III levels decreased and alpha 1at levels increased in all three groups during treatment. alpha 2M increased during cycle 3 in the Trinordiol and Gynatrol groups. CBG, PZP and SHBG levels increased in all 3 groups, the CBG and PZP increase being higher in the Marvelon group than in the Gynatrol group. Increases in SHBG levels were found in the order Marvelon > Trinordiol > Gynatrol. Plasma levels of complement factors B, C3 and C4 remained unchanged. It is concluded that the increase in alpha 1at partly compensates for the fall in AT III, that the rise in PZP presumably enhances fibrinolysis, and that LNg has higher anti-estrogenicity and androgenicity than DSG.