We have shown that the differential heat stability of alpha1PI may be utilised as the basis for a rapid, sensitive and reproducible ELISA assay of alpha1PI inactivation. In gingival crevicular fluid from periodontal disease patients, alpha1PI is mainly inactivated and the extent of this inactivation is much higher than in inflammatory fluids from other chronic diseases such as rheumatoid arthritis. This assay could be useful in monitoring the progression of periodontal disease.