The homozygote deficit of alpha 1 antitrypsin (alpha 1 PI-ZZ) in patients frequently results in a premature development of emphysema in the lung due to incomplete protection against proteases. An active inhibitor substitution appears to be useful. The presented study proves the biological effect of alpha 1 antitrypsin infused into 8 patients. The results were an activity increase of leukocyte elastase and trypsin inhibition in serum as well as doubling of alpha 1 antitrypsin in sputum. This therapeutical conception (with a dose of 60 mg/kg body weight/week) results in an efficient protection. Inhibitors specific for mucosa are not influenced. An improvement of lung function during 6 weeks of intravenous therapy was not achieved. The progressive destruction of lung parenchyma can be probably prevented, however.