Two complex enzymes were assembled that both converted C3 to C3b, one consisting of activated properdin (P), native C3, proactivator (PA) and proactivator convertase (PAase), and the other of nephritic factor (NF) and the same three cofactors. By maintaining a critical concentration of PAase, the P-C3 convertase and the NF-C3 convertase were shown to function efficiently without formation of the C3b-feedback enzyme. The former two enzymes are distinct from the C3b-dependent C3 convertase in that they utilize native C3 instead of C3b and PA in an apparently uncleaved form. The P- and NF-C3 convertase express maximal activity within approximately 10 min at 37 degrees C and decay with a half-life of 35 min at 37 degrees C, which is in contradistinction to the reported lability of the C3b-feedback enzyme. P- and NF-C3 convertases are inhibited by their product C3b, which may constitute a heretofore unknown control of the alternative pathway. A direct physical interaction of P with native C3 and C3b was demonstrated by agglutination of C3b-bearing erythrocytes and by agglutination inhibition. Bound C3b thus constitutes the only known receptor of P and may fulfill an important localizing function for P and the P-C3 convertase in vivo. Although P and NF form functionally similar enzymes, they act independently of each other and are apparently immunochemically unrelated proteins.