1. Methods for the separation from guinea pig serum in highly purified form of three of the components of complement are described. These substances are the so called mid-piece, end-piece, and 4th component.
2. Mid-piece has been separated as a euglobulin, with an electrophoretic mobility of 2.9 x 10–5 in phosphate buffer of ionic strength 0.2 at pH 7.7, and with a sedimentation constant of 6.4 x 10–13 in potassium chloride of ionic strength 0.2.
3. End-piece and 4th component were found together in a euglobulin fraction of serum which contained 10.3 per cent carbohydrate and had an electrophoretic mobility of 4.2 x 10–5 in phosphate buffer of ionic strength 0.2 at pH 7.7.