alpha-1-Antitrypsin phenotypes and serum levels were determined in 19 patients with juvenile periodontitis in order to test whether reduced periodontal resistance in this disease is caused by decreased serum protease inhibitory capacity resulting from deficient alpha-1-antitrypsin phenotypes. The phenotyping of the patients' sera was performed by thin-layer isoelectric focusing in polyacrylamide gel. The quantitation of alpha-1-antitrypsin was performed using the radial immunodiffusion technique. All 19 patients displayed the most common phenotype M. Twelve patients had the subclass M1M1, two the M1M2 and five and M1M3. The alpha-1-antitrypsin levels in the patients' sera were within normal limits. The results do not support the hypothesis that deficient production of alpha-1-antitrypsin is causally related to juvenile periodontitis.