In a variety of tissues (lymph node and glandular stroma), mast cells have been found in close and often intimate association with macrophages containing numerous ferritin-like particles in their cytoplasm and within cytoplasmic vacuoles (siderosomes). Phagocytic vacuoles in a given macrophage differed markedly. Some contained abundant Prussian blue-reactive material and others contained periodic acid-Schiff reactive substance at the light microscope level, and ultrastructurally some were filled with ferritin particles and others were not. Ferritin-like particles have also been observed occasionally in the mast cells associated with macrophages and even within the matrix of some of the granules in these mast cells.