Fig. (2) Heparin enhancing effect in the inhibition of thrombin by Serp-1. Thrombin at a fixed amount (0.002 NIH units) was incubated with Serp-1 at various amounts (A-H) in the presence or absence of heparin at 37 ^°C for one hour. The chromogenic substrate (S-2238) was added into the reaction and color development was monitored every hour for first 6 hours and at 12 and 24 hours thereafter (X axis). The residual thrombin activity (Y axis) was expressed as percentage of the control sample, which contained no Serp-1