In some cells exposed to horseradish peroxidase (HRP) for 30 sec and then immediately fixed some HRP has already entered the cell by endocytosis and has been removed from the early endosomes in carrier vesicles. In this picture such carrier vesicles have already docked at acidosomes that are themselves docked at a nascent digestive vacuole. Carrier vesicles will fuse with the acidosomes to add their content and membrane to the acidosomes. This is why two populations of acidosomes are often seen around the nascent phagosome and early DV-I, acidosomes that contain HRP because they have fused with HRP-containing carrier vesicles and those that have not fused with HRP-containing carrier vesicles. (See Allen, Ma and Fok, J. Cell Sci. 106:411-422, 1993). TEM taken on 2/24/82 by R. Allen with Hitachi HU11A operating at 75kV. Neg. 21,250X. Bar = 0.2µm.