The retinal pigment epithelium (RPE) from young albino rats was examined under light and electron microscope 2 1/2 to 16 h after intravenous injections of colchicine (0.5 mg per 100 gm body weight). One of the primary effects of colchicine treatment on the RPE is the appearance of extensive acPase-positive dense body aggregates in the Golgi area. The size of the aggregates and the disappearance of microtubules appear to be related to the length of time allowed for colchicine to react. Acid phosphatase (acPase) activity is localized on the dense body aggregates and is particularly evident at a light microscopic level, where it can be observed as darkly stained clumps. Dense bodies are only rarely seen in the immediate proximity of acPase-positive phagosomes in colchicine-treated animals. The results of these experiments suggest the existence of an intracellular transport mechanism for acPase-carrying dense bodies. This intracellular transport is inhibited by colchicine, being probably related to a block of microtubular assembly.