Benign and malignant cellular blue nevi have been studied cytopathologically, enzymatically and electron microscopically. Scarcely pigmented, Schwann-cell-like, spindle-shaped cells which characterize "cellular types" of blue nevus have been found to possess tyrosinase activity and can form premelanosome systems. These spindle-shaped cells can undergo melanization, at least in vitro, in the vesicles and cisternae of the smooth endoplasmic reticulum in the vicinity of the Golgi apparatus as well as in the intermediate vesicles and premelanosomes. It is concluded that the characteristic spindle-shaped cells of cellular blue nevus represent a different melanogenic activity but retain the cytological and enzymic properties of melanin synthesizing blue nevus cells. It is therefore proposed that in the future these spindle-shaped cells characterizing cellular blue nevus be considered "amelanotic blue nevus cells."