We measured thrombin-antithrombin III (TAT) complexes and antithrombin III (AT III) in amniotic fluid and blood plasma of 39 parturient women. TAT was measured by ELISA. Partigen plates were used for measuring the antigen of AT III, and activity was determined using chromogenic substrate. While AT III activity and AT III antigen were below 20% of plasma values, TAT concentration was 2–4 times higher in amniotic fluid than in blood plasma (over twice at the onset of labour, i.e. 54.2 ± 26.4 and 20.7 ± 10.7 ng/ml, and approximately 4 times after childbirth, i.e. 108.9 ± 49.2 ng/ml in amniotic fluid and 27.8 ± 13.2 ng/ml in blood plasma, respectively). We have concluded that thrombin generation and TAT formation in amniotic fluid are of significantly higher intensity than in plasma, and they increase during l