The laboratory control of low-dose heparin therapy is generally regarded as unnecessary. A laboratory study of the effects of low-dose heparin was performed using different methods: an amidolytic method and a method involving the inhibition of factor Xa in a coagulation test. Variations in partial thromboplastin time, recalcification clotting time, thrombin time and the plasma antithrombin HI levels were also studied. These tests were repeated (days 0, 1, 3, 8) in 27 women between the ages of 27 and 62 years who were undergoiing gynecological surgery. They received 5,000 IU of heparin either twice or thrice daily. There was no correlation between heparin levels in the blood and global clotting tests simultaneously performed. The plasma heparin levels varied between 0 and 0.15 IU/ml with both methods. A detectable heparin concentration on days 1, 3 and 8 was present in only half of the cases receiving the twice daily regimen. The plasma antithrombin III activity and concentration were not modified during treatment.