An assay was devised to quantitate acute intestinal inflammation based on the assessment of myeloperoxidase activity. Myeloperoxidase is an enzyme found in neutrophils and, in much smaller quantities, in monocytes and macrophages. Myeloperoxidase was solubilized with hexadecyltrimethylammonium bromide and myeloperoxidase activity was measured with a dianisidine-H2O2 assay. In neutrophil suspensions, myeloperoxidase activity was directly related to cell number down to as few as 500 cells. Myeloperoxidase activity was assayed in two animal models of inflammation: acetic acid-induced colitis in rats and Clostridium difficile enterotoxin-induced enteritis in hamsters. In both models, the activity of myeloperoxidase solubilized from the inflamed tissue was directly proportional to the number of neutrophils seen in histologic sections. Histologic evaluation of neutrophil accumulation was performed by counting the number of neutrophils in a histologic section 0.18 mm long and 5 micron thick. In both animal models, myeloperoxidase activity was linearly related to neutrophil number from 400 and 4000 cells/mm. Myeloperoxidase activity from chronically inflamed colon, in which both neutrophils and histiocytes were present, was directly related to neutrophil content. Histiocytes did not contribute significantly to myeloperoxidase activity. The determination of myeloperoxidase activity in the intestine is a simple biochemical assay that can be used to quantitate inflammation.