Summary:In sera of healthy persons (n = 50) and patients with a variety of diseases (n = 197) the two major proteinase inhibitors, αι-proteinase inhibitor (oti-antitrypsin) and ai-macroglobulin, were measured by two methods: a chromogenic (amidolytic) Substrate assay to assess the functional activities, and a laser nephelometric method to determine the immunoreactive concentrations of the respective proteins. The specific proteinase inhibitor activities defined s the number of inhibitor units per g inhibitor protein were calculated.The precision and accuracy of both assays were found to be similar, showing a satisfactory correlation of results for the sera of healthy persons (r ?= 0.916 for cta-macroglobulin and 0.988 for cti-proteinase inhibitor). In diseased individuals the correlation was lower than in normal persons (0.862 for a2-macroglobulin and 0.907 for αϊ-proteinase inhibitor). A poor correlation was obtained in patients with liver diseases (r = 0.586 for αι-proteinase inhibitor and 0.852 for ai-macroglobulin).Reference ranges were established for functional and immunological concentrations and for specific inhibitor activities, respectively. Normal values followed a Gaussian distribution.In patients with various diseases including those with acute phase response, the specific inhibitor activities of ctrproteinase inhibitor re reduced significantly; this is because inhibitor activity shows a smaller relative increase than immunoreactivity. Among the various diseases, no significant differences were noted. The specific inhibitor activity of az-macroglobulin changed significantly only in patients with carcinoma, liver diseases and trauma. Follow up of some patients shows also intraindividual Variation of specific proteinase inhibitor activities. Amidolytische und immun-nephelometrische Bestimmung von a/-Proteinase-Inhibitor und