This paper describes a one-step light microscopy method for demonstrating the antigen contents of unequivocally identified mast cells. It is based on the differential metachromatic properties of proteoglycans, mostly heparin and chondroitin sulfate, and 1-naphthol in the presence of toluidine blue in an acidic medium. Proteoglycans occur in all mast cells and 1-naphthol is used to demonstrate the peroxidase activity of the sections treated by the horseradish peroxidase-labeled avidin-biotin complex method for antigen detection. Granules containing proteoglycans present the classical metachromatic reaction by appearing purplish-red, while granules containing antigen appear a brilliant green. When both types of granules are distinct inside the cell, single- and double-stained cells can be accurately separated and counted. We hope that this new procedure will contribute to a further identification of mast cell mediator contents and to a better understanding of the physiology of this cellular population.