C3 nephritic factor (C3NeF) defined by the capacity of nephritic serum and its fractions to initiate loss of the B antigen of C3 in normal serum was purified from the serum of three different donors and shown to function by stabilization of membrane-bound and fluid phase alternative pathway C3 convertase. C3NeF converts cell-bound C3B̅ sites in a dose-related manner to C3B̅(NeF) sites, which exhibit and approximate 10-fold increase in half-life. The linear relationship between the C3NeF input and the residual hemolytic sites on EAC43B present after incubation for 20 min at 30°C, during which labile C3B̅ sites, have decayed, indicates that the number of residual C3B̅ sites is directly related to the dose of C3NeF. The capacity of C3NeF to stabilize the C3B̅ convertase in a temperature- and dose-dependent manner, which is independent of binding or consumption of C3NeF, in a fluid phase reaction mixture of ¹²⁵I-B, ¹³¹I-C3, and D̄ permits isolation of a 10S complex containing radiolabeled C3 and B and exhibiting C3 convertase activity on an exogenous C3 source. Thus, the stabilizing effect of C3NeF is not limited to membrane-bound C3B̅ but is also sufficient to permit recovery of a fluid phase C3 convertase formed during the interaction of C3, B, and D̄.