Factor D is an active serine esterase capable of specifically cleaving factor B in the presence of C3b and thereby generating the active C3- and C5-convertase C3bBb. In the absence of C3b when nephritic factor (NF) or activated properdin (P̄) is available, D̄ appears to have a different function. Native C3, B, and NF or P̄ form a complex which, dependent on traces of D̄ has C3 cleaving, C3b generating activity. B seems to remain in this complex in its uncleaved form [Schreiber et al., J. Exp. Med. 142, 760 (1975)]. This suggested to us that D̄ might have other than hydrolytic activities, the more so as we found it difficult to completely block its functional activity in the generation of B-dependent enzymes by the esterase inhibitor, DFP.