Conventional procedures for determining Km or Ki values generally employ subsaturating concentrations of substrate and competitive inhibitor; however, this is impractical in some cases if a significant percentage would be enzyme-bound or metabolized. Applications employing high and competing concentrations of substrates and inhibitors are described. Ki or Km values can be calculated from the equations. Ki = [I]/((V0)/V - 1)(1 + [S]/Km)) Km = [S](V0/V - 1)/(1 - V0/V + [I]/Ki), where V0 and V are the rates in the absence and presence of inhibitor. This is illustrated by measurements of inhibition of human urinary kallikrein by synthetic peptides.