The rate of elimination of injected antithrombin III (AT III) concentrate was investigated by measuring plasma AT III levels in two control subjects and four patients treated with continuous intravenous infusion of heparin. No significant differences were noted in the disappearance rate of injected AT III between controls and patients receiving heparin for several days. Their half-time of AT III elimination was 8.4--13.6 h. The patient in whom AT III concentrate was injected at the start of heparin therapy showed a very rapid decrease of AT III activity with a three-fold increase in the elimination rate constant. Inactive, modified AT III protein present in purified AT III concentrate was found in the circulation of all injected patients. No evidence was obtained that this altered AT III was eliminated more rapidly than the unaltered AT III or that it was subjected to accelerated clearance during heparin therapy.