A sensitive assay for the functional activity of complement Factor I is described. This is based on its third proteolytic clip whereby Factor I cleaves cell-bound iC3b to cell-bound C3dg and soluble C3c, thereby abolishing conglutination of the cells. Factor H is required as a co-factor for Factor I activity. Because of the low affinity of iC3b for Factor H, the assay needs to be performed at low ionic strength. This assay is easier to perform than those based on the conversion of C3b to iC3b (the first two Factor I clips), there being no need for the unstable intermediate EAC142 or for purified C3.