Tissue mast cells have been a source of great curiosity since their conclusive description by Ehrlich over 80 years ago. They have been implicated in a vast majority of connective tissue reactions both of physiologic and pathologic nature. In some species at least, mast cells have been implicated in the production, storage and release of potent cardiovascular substances including heparin, serotonin and histamine.The present report is directed particularly toward man. Evidence is presented suggesting that human tissue cells previously lumped into one large group on the basis of granular metachromasia and termed tissue mast cells, represent at least two morphologically distinct cellular types. Difference in function of these cells may be great. There is evidence that one type may be a local tissue source for epinephrine, norepinephrine, or both. It is suggested that mast cells as a whole may represent a spectrum of cell types having numerous differences in ultrastructure and function. It is believed that their continued study in various physiologic and pathologic states should prove fruitful.