C3NeF, recognized by its capacity to stabilize the cell-bound amplification convertase, C3b,Bb, was purified from sera of three patients with hypocomplementemic glomerulonephritis by QAE A-50 Sephadex and SP C-25 Sephadex chromatography, affinity for the amplification convertase and QAE A-50 Sephadex chromatography. C3NeF activity exhibited heterogeneity during cation exchange chromatography on SP C-25 Sephadex and on isoelectric focusing, with forms having peak pI's ranging from 8.35 to 8.9 C3NeF was further purified by interaction with purified B, D̄, and C3 to form fluid phase (C3b,Bb(C3NeF) which sedimented as a 10S complex on sucrose density gradient ultracentrifugation. The C3NeF was recovered upon decay of the isolated convertase and was separated from C3b and Bi chromatography on QAE A-50 Sephadex (Daha, Austen and Fearon, J. Immunol. Sept. 1977). The final preparations exhibited specific functional activities of 45 to 50 units/µg protein and were radiolabeled with 125I without decrement in function.