Crystal-containing cells are found with regularity in the cecal epithelium of the mouse. The crystal-containing cells may contain aggregated rhomboidal crystals or membrane-bound semicrystalline granular inclusions which demonstrate a 60 lamellar periodicity. Treatment of mice with actinomycin D increased the number of crystal-containing cells and the number of crystals held within them. The increased number of crystal-containing cells is shown to arise not from the existing cell types of the epithelium, but from a migratory cell type which invades the cecal mucosa from the lamina propria. An intermediate cell type possessing morphological similarities of a granulocyte and the crystal-containing cell is suggested as the precursor to the crystal-containing cell.