To identify the nature of the IL-8 inhibitor(s) in gingival crevicul fluid (GCF).(1) 14 GCF samples were collected from 13 adult periodontitis (AP) patients, and 9 samples were taken from 8 healthy subjects. Each GCF sample was divided into two aliquots. A serine protease-specific inhibitor-PMSF was added to one aliquot and PBS added to the other aliquot as control. ELISA was used to measure IL-8 level in the samples. (2) 41 GCF samples were collected from 15 AP patients and indirect ELISA was performed to detect the anti-IL-8 IgG antibody.(1) IL-8 level in the PMSF-added samples were significantly greater than that in the control group(3.01 mg/L +/- 5.79 mg/L vs 0.05 mg/L +/- 0.15 mg/L, P < 0.001). (2) The mean value of anti-IL-8 IgG in GCF was greater than that of negative control +3 x s.A serine protease which can "cleave" IL-8 exists in GCF and GCF from AP sites contain auto-antibody against IL-8.