This review article acknowledges the pioneering contribution of William Bate Hardy in shaping the concept of mast cell heterogeneity. In two outstanding papers, published in 1894 and 1895, he focussed on the 'wandering cells' (the modern leucocytes) in different mammalian species and distinguished two types of granular basophil cells, i.e., the coarsely granular basophil cells and the splanchnic basophil cells. These corresponded to the populations of connective tissue-type and mucosal mast cells, respectively, described 70 years later by Enerbäck in rodents. Among the coarsely granular basophil cells, he also differentiated those cells which populated the serosal cavities - the so-called coelomic coarsely granular basophil cells - from the common coarsely granular basophil cells, which were localized in the connective tissues. He stated that the granular basophil cells presented with different morphological and histochemical characteristics in diverse animal species as well as at different anatomical sites. Remarkably, he performed a series of functional experiments on the basophil cells as well as the other wandering cells, and suggested the view that different granular basophil cells might express functional specializations.