An improved procedure for the simultaneous purification of C3 and of β1H globulin from human serum has been developed. With this procedure up to 400 mg of highly purified C3 can be prepared from 800 ml of serum (25–35% overall yield). The final stage of this procedure involved chromatography on an ‘anti-impurity’ column to remove traces of contaminating proteins, rendering the product suitable for functional as well as structural studies. Although primarily developed for C3, with this procedure, β1H is obtained as a by-product in high yields and better than 95% pure. The behaviour of other complement proteins, especially those that tend to co-purify with C3 (e.g. C5) has been followed throughout the procedure. A preliminary chemical characterization of C3 purified by this method has been carried out.