A one-step sandwich enzyme immunoassay (EIA) system for human matrix metalloproteinase 8 (MMP-8, neutrophil collagenase, EC 3.4.24.7) has been established with a pair of monoclonal antibodies prepared against the zymogen of MMP-8 purified from human neutrophils. MMP-8 in samples simultaneously reacted with both solid-phase and peroxidase-labeled antibodies. Sensitivity of this EIA system was 0.34 micrograms/l (5.7 pg/assay) and linearity was obtained between 0.5 and 500 micrograms/l (8.3-8300 pg/assay). The EIA system recognized both precursor and active forms of MMP-8 but not MMP-8 complexed with tissue inhibitors of metalloproteinases. There was no difference in the MMP-8 levels between the plasma samples from patients with rheumatoid arthritis or osteoarthritis and those from healthy subjects (median 6.2 micrograms/l, range 1.5-28 micrograms/l). However, the level in synovial fluids from patients with rheumatoid arthritis (median 345 micrograms/l, range 84-2860 micrograms/l) was shown to be higher than that from osteoarthritic patients. MMP-8 levels in human whole saliva from patients with periodontal diseases (median 282 micrograms/l, range 0-1420 micrograms/l) were also significantly higher than those from clinically healthy subjects (median 25 micrograms/l, range 0-100 micrograms/l). Immunoreactivity analyses showed that MMP-8 species in normal human plasma exists as a precursor but not as a complex form with tissue inhibitor of metalloproteinases (TIMP)-1 or TIMP-2.