BF Tack and J. W. Prahl* abstract: The third component of complement has been purified from fresh human plasma employing an initial fractionation with poly (ethylene glycol) followed by sequential depletion of plasminogen by affinity adsorbents, chromatography on diethylaminoethylcellulose, gel filtration on agarose, and batchadsorption/desorption on hydroxylapatite. Final recoveries of C3 were 33% of the initial protein, as quantitated by radial immunodiffusion, and 31% of the initial hemolytic activity. Apparent homogeneity is indicated by immunological criteria and by polyacrylamide gel electrophoresis. A partial specific volume of 0.736±0.003 mlgm'1*** was determined for C3 by the mechanicaloscillator technique.“Low speed” sed-imentation equilibrium yielded an apparent weight average molecular weight for the protein of 187 650±5650. Based upon this molecular weight, a molar extinction coefficient of 1.82 X 105 1. mole'1 cm-1 at 280 nm was calculated from

. L he complement system of vertebrates functions as a hu-moral biological effector following antigen-antibody inter-action. Complement shares many similaritieswith the complex of coagulation proteins, which may reflect the similar evolu-tionary pressures operative on both systems. Both the com-plement and the coagulation systems are characterized by their multimolecular complexity, activation being achieved in a sequential or “cascade” process, with the presence of inhibitors and inactivators to achieve control and fine tuning of the ac-tivation process, and, finally, the existence of alternative pathways of activation and feedbackcontrol (Miiller-Eber-hard, 1975; Prahl, 1976).