To study the distribution of two different phenotype mast cells in atherosclerotic lesions of human carotid arteries, 32 samples of endarterectomy and 10 autopsies of normal carotid specimens were fixed and embeded in paraffin, sections were stained with hematoxylin-eosin, toluidine blue,monoclonal antibodies against the two major proteases of mast cells(tryptase and chymase) and monoclonal antibodies against CD-3 of T lymphocytes and CD-68 of macrophages. The quantity,distribution and degranule of two different phenotype mast cells in atherosclerotic lesions of human carotid arteries were investigated,and the ratio of mast cells to T lymphocytes and macrophages in atherosclerotic lesions was calculated. The density(cells/mm 2 ) of mast cells in shoulder region of plaque(6 3?4 0) was much higher than that in normal intima,cap and core region(1 3?2 3,2 3?2 6,2 2?3 7) .The proportion of chymase-containing mast cells to tryptase-containing mast cells was 0 36,0 51,0 47 and 0 39 respectively in normal intima,cap,core and shoulder region of plaque.The propotion of degranulated mast cells was the highest in shoulder region of plaque(65%).The ratios of mast cells to T lymphocytes and to macrophages were 0 25 and 0 14 respectively in shoulder region of plaques.Light microscopic studies of mast cells revealed that mast cells were more commonly observed in thrombus and around calcification and blood vessels of plaque.Mast cells play a role in the formation of plaque,and participate in the destabilization of plaque.