The aim of the study was to determine the effect of the antithrombin concentration on the anticoagulant response to heparin in vitro. Pooled plasma was depleted of antithrombin using heparin Sepharose. Unfractionated heparin (final concentration between 0 and 1 U/ml) was added to plasma with different antithrombin concentrations, and the activated partial thromboplastin time (APTT) was determined. Plasma with an antithrombin concentration over 0.27 U/ml shows a similar response to heparin as normal plasma; at lower levels the response is diminished. Even in plasma fully depleted of antithrombin, the APTT can be prolonged to a therapeutic level, although the amount of heparin needed is twice as high. At antithrombin concentrations over 0.27 U/ml, heparin is the major determinant of the anticoagulant effect.