Macrophages and mast cells in masseter muscle from normal and dystrophic mice were studied by light and electron microscopy. Acid phosphatase activity and FITC-dextran were used to identify and describe macrophages. Toluidine blue was used as a marker for mast cells. In dystrophic muscle, the number of macrophages was greatly increased and they contained large amounts of phagocytozed material. In normal muscle, mast cells were rarely identified whereas dystrophic muscle contained many mast cells which were often in close contact with macrophages.