Abstract: Mast cells in the intestinal mucosa (MMC),in the connective tissue of the intestinal submucosa (CTMC) and in the thymus medulla (TMMC) from four 1-month-old Large White pigs were observed under transmission electron microscopy.No evidence shows significant ultrastructural differences among porcine MMC.CTMC and TMMC.Porcine mast cells contained characteristic granules of uniform size,with a circular or oval outline and bounded by a unit membrane.The cytoplasmic granules contain amorphous granular material but the electron density of these matrix material varied from high to low.One or two lower dence cores were found in the center or eccenter of some granules.Based on electron density of the granule matrix,the granules could be classified into three types (Type Ⅰ,Ⅱand Ⅲ).The Type Ⅰ,Ⅱ and Ⅲ granules contain high,less high and low electron dense amorphous granular matrix,respectively.