In vitro and in vivo studies were carried out to determine whether a standardized laboratory procedure, the activated (kaolin) partial thromboplastin time (PTT), could be used to regulate heparin therapy. In vitro, increasing heparin concentrations caused progressive prolongation of both the Lee-White (L-W) coagulation time and the activated PTT. Comparison of the activated PTT with the L-W coagulation time in 78 patients revealed an approximate linear correlation between the two tests, with corresponding prolonged values for each test found in almost all heparinized patients. Our observations suggest that the activated PTT test procedure can be used to regulate heparin anticoagulation. The major advantage of this test is that it can be performed in the laboratory by trained technicians and completed within a few minutes.