Serpins inhibit proteinases by a branched pathway, in which an intermediate serpin-proteinase complex can either form a stable covalent serpin-proteinase complex or produce reactive center cleaved serpin in a substrate reaction. It was tested whether these competing reactions could be regulated for the serpin C1-inhibitor by ligand binding. C1-inhibitor bound to type IV collagen, laminin, and entactin. Type IV collagen (10 microg/ml) caused an increase in the stoichiometry of inhibition for C1s inhibition by C1-inhibitor to 1.48 from 1.09 in the absence of ligand. A dose-dependent increase in the stoichiometry up to 1.27 in the presence of 100 microg/ml heparin was also observed. At low ionic strength the stoichiometry increased to 2.55. These data provide the first report that C1-inhibitor can bind to type IV collagen and also show that C 1-inhibitor can be regulated by ligand binding.