The third component of complements (C3) has been purified from plasma (blood type: B) by sequential employment of QAE-Sephadex A-50 chromatography, 20% (w/v) PEG-3,350 precipitation, heparin-Sepharose CL-6B, and hydroxylapatite chromatographies. Final recovery of C3 was 57% as quantitated by single radial immunodiffusion (SRID), and purification fold was 78. The SDS-PAGE indicated that the purified C3 was homogeneous. Mercury ions or low concentration of magnesium ion showed stimulatory effects on the hemolytic activity of C3, which were inhibited by EDTA or high concentration of reducing agent. Degradation of C3 appeared to be selectively enhanced by several factors, while lysate of sheep red blood cells (SRBC) inhibited it.