Levels of anti-Xa and anti-thrombin III activity have been determined in a series of patients clinically definable as "thrombophiliac" and of women undergoing oestroprogestinic contraceptive treatment. The levels of plasma Co-Ep activity for potentiating anti-Xa activity on the part of differing in vitro added doses of Ep have also been determined. The fact that anti-Xa fell in more marked a fashion than the corresponding anti-thrombin III may be the humoral equivalent of a condition of plasma hypercoagulability. Treatment with coumarin derivatives may bring about an appreciable increase anti-Xa plasma levels. The significance of these variations is discussed and the sensitivity of the anti-Xa method for monitoring small dose heparin therapy, i.e. based on antithrombotic rather than anti-coagulant effect, is confirmed.