The number of one subtype of mast cells (formalin fixation, toluidine blue staining), cells of the lamina propria, and intraepithelial lymphocytes were counted in the intestinal biopsy specimens of 14 children with treated celiac disease following a single challenge with gluten. The serum neutrophil chemotactic activity was measured at 0, 1, 3, 5, and 24 h after challenge. There was no significant change in the number of intraepithelial lymphocytes, but the biopsy samples obtained at 5 h showed a marked increase in the inflammatory cells of the lamina propria and a significant decrease in the number of mast cells. A pronounced decrease was present at 3-5 h in the number of eosinophil cells in the blood. The neutrophil chemotactic activity of sera showed a significant increment in 10 of 14 patients. The intestinal permeability of patients became abnormal, as detected by the increased absorption of lactulose. These findings suggest that degranulation of mast cells may be involved in the pathogenesis of the small intestinal mucosal injury in children with celiac disease.