alpha 1-Antitrypsin (AAT) has been purified from human plasma supernatant A (equivalent to COHN fraction II + III) by a large-scale chromatographic procedure involving anion-exchange adsorption on DEAE Sepharose CL-6B fast flow and size-exclusion chromatography on Sephacryl S-200. Before freeze-drying, the liquid concentrate was heat-treated at 60 degrees C for 10 h to reduce the risk of transmission of blood-born viral diseases. Using this procedure, AAT is recovered with 80-90% purity in 65-75% yield from supernatant A. The heterogeneity of AAT is preserved across the purification steps. In addition, purified AAT exhibits inhibitory activities against trypsin and elastase equivalent to that of the serum protein. The mean association rate constant for elastase was found as high as 2.15 X 10(5) M-1 s-1. Thus, purifying active AAT from supernatant A contributes to improving the availability of this protein which may be potentially useful in the treatment of hereditary emphysema.