A qualitative microspectrophotometric detection method for heparin in situ has been developed, using data obtained previously with a model system of polyacrylamide films containing pure glycosaminoglycans (Tas & Roozemond 1973, and Tas 1975). This technique, based on the unique metachromatic properties of heparin with Toluidine Blue O in glycerol, has been worked out with rat peritoneal and mesenteric mast cells. After equilibration of the stained, air-dried cells in glycerol (for some days), the specific peak of the heparin-Toluidine blue O complex (as found in the model experiments at about 515 nm) could be recorded. It was found that the method can be used to detect unequivocally the presence of heparin in cells, even if they also contain up to 75 mole percent of other, lower sulphated-glycosaminoglycan. Otherwise the method might yield some information about the degree of sulphation of the heparin concerned. With this technique the presence of heparin has been proved in young and adult rat mast cells and for the first time now directly in normal human mast cells and normal human basophilic granulocytes.