PIP: The authors examine the variations of plasma factor 13 (PF13) levels during the physiological cycle and estroprogestative contraception. Blood samples were taken from 8 women aged 21-26 who had used a combined estroprogestative oral contraceptive (OC), ethinyl estradiol 0.003 mg, levonorgestrel 0.15 mg, and from 10 healthy women aged 23-29, normally ovulating. Each subject underwent 4 venopunctures during the same month. The detection of PF13 levels from the 10 normally ovulating women resulting in the following mean values were found: 1) proliferative phase, day 6-10, 92%; 2) periovulatory phase, day 13-15, 67.2%; 3) secretive phase, days 20-23, 85%; and 4) premenstrual phase, day 27-29, 89.4%. There was a highly significant decrease of PF13 during periovulatory phase when compared with the other phases (p0.001). The detection of PF13 levels for the 8 women using OCs was: 1) end of 1st week from bleeding, day 7-8, 112.5%; 2) end of 2nd week, days 13-15, 113.1%; 3) end of 3rd week, days 20-22, 113.1%; and 4) end of 4th week, days 27-29, 113.1%; for the latter group no significant differences were found. The mean values resulted higher for the women using OCs in every serial sample. These results suggest a possible action of steroid hormones on PF13 levels and through it on the last step of coagulation process, the fibrinostabilization.