An endogenous ACE activator has been revealed. Neutrophil-enriched human leucocyte preparations released in isotonic media a relatively thermostable factor, capable of increasing the angiotensin-converting enzyme activity 1,6 - 2,0-fold. Activator would not pass through a 30 kDa cutoff membrane. Data on the presence of the activator in vivo were obtained in studies of protein-free serum fractions of patients with a hereditary CI-esterase inhibitor deficiency. It was suggested that the neutrophil-released ACE activator could have a role in the modulation of the inflammatory response.