A two-site enzyme immunoassay for slow moving protease (SMP) of human gastric mucosa has been developed. The detection limit of SMP was 50 pg/tube and the optimal assay range was 0.5 to 50 ng/tube. This assay system made it possible to measure a trace amount of gastric mucosal SMP in endoscopic biopsy-specimens. Mean gastric mucosal SMP contents of the antrum, the angles, and the distal and proximal corpuses were 4.5, 3.7, 2.6, and 2.5 micrograms/mg protein, respectively. The antral mucosa contained a larger amount of SMP than the gastric mucosa of other area. The present enzyme immunoassay system is sufficiently sensitive for the clinical study of SMP in gastrointestinal tract.