Inhibition of prothrombinase by antithrombin III (ATIII) and heparin was investigated in a continuous-flow system. Phospholipid-coated capillaries, containing phospholipid-bound factor Xa and factor Va, were perfused with 1.0 mumol/L prothrombin and 0.5 nmol/L factor Va. At 25 degrees C and a flow rate of 32 microL/min (shear rate 28 seconds-1) the steady-state rates of prothrombin conversion depended linearly on the surface concentration of prothrombinase up to 2 fmol/cm2. The rate of thrombin generation was 952 +/- 43 (SE) mol/min/mol prothrombinase. When ATIII was included in the perfusate for 10 minutes, the free thrombin concentration at the outlet of the capillary was markedly reduced: a 50% neutralization was obtained at 0.7 mumol/L ATIII. However, the prothrombinase activity was not inhibited, as could be established after a subsequent perfusion with prothrombin and factor Va. At an ATIII concentration typical of normal plasma (2 mumol/L) a slight neutralization of prothrombinase was observed: 10% neutralization following a 10-minute perfusion. During a perfusion with ATIII in the absence of prothrombin, or in its presence with hirudin (2 mumol/L) also included in the perfusate, a more pronounced neutralization of prothrombinase was observed: 40% residual activity was obtained after a 10-minute perfusion. From this observation the suggestion comes forward that thrombin, continuously produced at the surface, consumes ATIII in the boundary layer. In this case the true ATIII concentration in the vicinity of surface-bound prothrombinase will be but a small fraction of the initial ATIII concentration in the bulk fluid. Unfractionated heparin and an ultra-low molecular weight heparin (pentasaccharide) did enhance the ATIII-dependent neutralization of prothrombinase, but to a much lesser extent than observed with small unilaminar phospholipid vesicles as the catalytic sites for prothrombinase assembly. The findings reported here support the notion that regulation of prothrombinase by heparin under in vivo conditions occurs at the stage of its formation, ie, through inhibition of free factor Xa and/or the generation of factor Va, rather than by direct inhibition of the prothrombinase activity.