SummaryChromogenic peptide substrates were used to measure plasminogen, c*2-antiplasmin, and antithrombin-III on a number of days during one single cycle of a combined oral contraceptive regime in 8 volunteers not taking a contraceptive before. Fibrinogen was measured with a clotting method. Blood samples were taken immediately before treatment was started and on days 2, 4, 7, 9, 11, 14, 16, 18, and 21. Plasminogen levels increased significantly from day 4 on during the whole cycle. The fibrinogen values showed the same trend. Statistically significant increases were found from day 4 to day 18. The ayantiplasmin values we’re lower on day 2 and antithrombin-IIi levels were only significantly decreased on days 9 and 21.