A method for the preparation of highly purified C′1 (“mid-piece”) from human serum is described. Human C′1 is characterized as a euglobulin with an electrophoretic mobility of 2.9 × 10−5 in veronal buffer of pH = 7.8 and ionic strength 0.1. Examined in the Svedberg oil-turbine ultracentrifuge in a 1.2 per cent protein and 3 per cent NaCl concentration, the C′1 preparation was shown to contain a component which had a sedimentation constant of 6.9 S when corrected to water at 20 C, and which constituted 70 per cent of the material.