Electronmicroscopic-cytochemical investigations of human and guinea pig (normal and irradiated) melanocytes as well as junctional nevus cells (benign juvenile melanoma) have revealed that 1) premelanosomes of pigment cells contain acid phosphatase. This confirms previous observations. The enzyme is attached to the internal membranes which represent the sites of tyrosinase activity and melanin deposition. The external (delimiting) membrane of premelanosomes is devoid of acid phosphatase. 2) Golgi cisternae and Golgi vesicle also contain acid phosphatase and, occasionally, the enzyme can be visualized in the endoplasmic reticulum.