A technique using alkaline phosphatase histochemistry on routine sections of four jejunal biopsy specimens and one necropsy sample was applied to show that alkaline phosphatase activity, normally present in the brush border, occurs in the enterocytes of patients with microvillus inclusion disease. Sections were cut at 5 micron, mounted on to glass slides, and dried overnight at 37 degrees C before staining for alkaline phosphatase activity by the indoxyl phosphatase nitro blue tetrazolium method. Incubation periods amounted to 10 minutes for biopsy specimens and 30 minutes to one hour for necropsy samples. The demonstration of alkaline phosphatase activity in routinely processed biopsy specimens provides an effective, quick, and definitive test in the diagnosis of microvillus inclusion disease without recourse to electron microscopy.