Factor H, a regulator of complement activation, contains 20 short consensus repeat (SCR) domains common among the family of C3b/C4b-binding proteins. Chinese hamster ovary cells transfected with cDNA corresponding to the N-terminal tryptic fragment of factor H (containing SCR 1-5 and part of SCR 6) secreted protein with cofactor activity for factor I-dependent cleavage of C3b. A series of deletion mutants, each lacking one of the first five SCR, were constructed, and the supernatants of transfected Chinese hamster ovary cells were tested for cofactor activity. Supernatants of Chinese hamster ovary cells transfected with SCR 1, SCR 4, and SCR 5 deletion mutants retained cofactor function, although the SCR 1 deletion had reduced cofactor activity. Deletion of SCR 2 or 3 totally abolished cofactor activity. Expression and functional analysis of SCR units 1-3, 2-3, and 2-4 demonstrated that the SCR 1-3 unit is sufficient for cofactor activity, but SCR 1-4 is required for full activity. For assays involving cell protection, a construct linking SCR 1-5 to the glycosyl-phosphatidylinositol anchor of decay-accelerating factor was prepared, and stable transfectants were obtained. These cells were protected against complement-mediated cytotoxicity, similarly to decay-accelerating factor- and membrane cofactor protein-transfected cells. These studies define the complement regulatory domains in factor H and suggest that the general complement functional unit for C3 convertase regulation involves three or four consecutive SCR units.