The Gaucher cell results from the accumulation of excessive glucocerebroside in cells of the monocyte-macrophage system. It is characterized ultrastructurally by the presence of cytoplasmic inclusions which consist of tubule-like structures measuring 130 to 150 Ao in diameter. Utilizing freeze fracture and x-ray diffraction techniques these structures appear to be bilayers measuring 6 nm in thickness, 40 to 60 nm wide and up to 600 nm in length. The Gaucher cell contains isoenzyme 5 of acid phosphatase which is resistant to prior incubation in tartaric acid. The bone marrow smears from patients with Gaucher's disease contain monocytes and monocytoid cells which contain inclusions which are acid phosphatase positive and resemble the acid phosphatase positive inclusions in the Gaucher cells. Ultrastructural studies of monocytes from Gaucher patients demonstrated membrane-bound inclusions containing tubule-like structures identical to those in Gaucher cells. These monocytic cells appear to represent precursors of Gaucher cells.