Tissues from three siblings with inclusion-cell (I-cell) disease (a 16 month old boy and two fetuses aborted at 20 and 18 weeks) were investigated histologically, histochemically and ultrastructurally. The lymphocytes, fibroblasts, endothelial cells, epithelial cells and histiocytes of various organs were affected. The cells had many intracytoplasmic vacuoles, which showed positive staining with colloidal iron, periodic acid-Schiff (PAS), alcian blue, and Sudan III and IV. Ultrastructurally, the cells contained various inclusion bodies, showing vesicles, granules, flocculent material, amorphous electron-dense globules and myelin structures. The amounts and ultrastructural features of the inclusion bodies differed among the various kinds of cells.