The activity of acid phosphatase in the principal cells of the guinea pig epididymis was studied histochemically. The enzyme activity was localized in the Golgi and apical regions in segments 1–4. In segments 5–7, the enzyme activity was distributed throughout the entire supranuclear cytoplasm. There was a gradual increase of acid phosphatase activity from segments 1–7. A possible function of acid phosphatase in the epididymis is disc