Four pinealomas of the two-cell type (atypical teratomas) were investigated by electron microscopy. They all showed the same unique histological ultrastructure. The lymphocyte-like cells did not differ ultrastructurally from lymphocytes. Many plasma cells with well-developed rough surfaced endoplasmic reticulum were also seen. The small cells and the plasma cells were considered to be derived from blood cells. A variety of the large cells in the process of differentiation were observed. They had a large and ovoid nucleus with uniform granular chromatin and contained one or more prominent nucleoli. The large cells had many dilated cytoplasmic cisternae, numerous glycogen granules of about 250–300 A in diameter and annulate lamellae. Ultrastructurally, the pinealoma of the two-cell type resembled the seminoma of testis which is of germ cell origin. This suggests that the pinealoma may be of germ cell origin.