The mast cells in the thymus, lymph nodes and spleen of rhesus monkey ( Macaca mulatta ) were observed by transmission electron microscopy and histochemical staining method using toluidine blue and alcian blue-safranin The results showed that they exhibited heterogeneity in shape, size, staining property and ultrastructure These mast cells varied in shape and size in the parenchyma and stroma of the thymus, in the stroma of lymph nodes and spleen, but were small and round in the parenchyma of the lymph nodes and spleen All the mast cells exhibited metachromasia with toluidine blue staining, but were almost all blue (alcian blue positive), rarely showed red (safranin positive) or mixed red-blue (double positive) with alcian-safranin staining Under transmission electron microscope, they were smooth or possessed a few processes on the surface, containing abundant organelles, also homogeneous secretory granules of high and moderate electron density in the cytoplasm