The catalytic domains of activated C1r and C1s, comprising the C-terminal region of the A chain (gamma), disulphide-linked to the B chain, were obtained by limited proteolysis of the native proteases with chymotrypsin and plasmin, respectively, and studied by small angle neutron scattering. For activated C1s (gamma-B), a molar mass of 45,000 +/- 5000 g/mol, and a relatively large radius of gyration (Rg) of 28 +/- 1 A were determined, excluding a single globular domain. The corresponding values for activated C1r (gamma-B)2 (90,000 g/mol, Rg = 34 +/- 1 A) are consistent with a dimer involving the loose packing of two (gamma-B) subunits. Various models of the dimer are discussed in the light of neutron scattering and other data.