This invention relates to the field of determining, assaying or quantifying activity of components of the complement system. More particularly, the invention relates to methods for detecting the presence or level of activity in a sample of mannan-binding-lectin associated serine proteases (MASPs) or complexes of such proteases with lectins and to detection of the particular lectins themselves. Provided is a method for determining the activity of a MASP in a sample, comprising incubating the sample with a pro-urokinase comprising at its activation site the consensus sequence Arg/Leu/Gly-Yyy-Arg/Lys-Ile/Leu/Val-Zzz-Gly-Gly cleavable by a MASP, wherein Yyy can be any amino acid and Zzz is preferably an aliphatic amino acid, and determining proteolytic activation of said pro-urokinase.