Properdin exists in at least three functionally different forms. The form which is isolated by the most rapid (two-step) procedure from fresh serum we designate native properdin (nP). Purified nP is characterized by its inability to bind to solid phase C3b, to stabilize the labile C3/C5 convertase, to form a soluble C3 convertase in serum, and by its capacity to reconstitute P-depleted serum with respect to C3 consumption by zymosan or inulin. The second form of P, designated pre-P, is characterized by its ability to bind reversibly to solid phase C3b, to stabilize the labile C3/C5 convertase formed with multiple C3b molecules, and to reconstitute P-depleted serum in a manner analogous to nP. The third form is active P, referred to as P̄. It is characterized by its ability to bind irreversibly to solid phase C3b, to stablize the labile C3/C5 convertase as well as the single C3b C3 convertase and to assemble the soluble C3 convertase when added to serum independent of zymosan or inulin.