Fig. (4) Heparin-dependent inhibition of thrombin by Serp-1. Thrombin at a fixed amount (0.002 NIH units) was incubated with Serp-1 at three non-inhibiting amounts (38, 75, and 150 ng) with increasing units of heparin (X axis). The experiment conditions were similar to those in Fig. 2 except color generation was monitored at 17 hour after the substrate was added into the reactions. The residual thrombin activity (Y axis) was expressed as percentage of the control sample, which contained no Serp-1.