Mast cells in normal human skin and in lesional cutaneous tissue from two patients with mastocytosis were quantified with the use of a morphometric point counting technic in combination with the mast cell-specific stain, conjugated avidin. Mast cells in normal human flank skin occupied a relatively small percentage of the dermal volume, with a mean value of 0.40% (+/- 0.14 SD), while a marked increase in mast cell content was demonstrated in cutaneous lesional tissue from two patients with the macular and nodular forms of mastocytosis (2.5% and 64.3%, respectively). In a comparative study with the use of morphometric analysis, conjugated avidin proved objectively to be as good as toluidine blue, and subjectively superior to this stain for mast cell identification in both human and murine skin. The combination of morphometric point counting and the conjugated avidin stain provides a useful tool for establishing the diagnosis of mastocytosis and is applicable to a variety of experimental systems.