Isolated human skin mast cells (HSMC) were prepared and cultured overnight before functional and electron microscopic studies. Mast cell suspensions were examined after stimulation with anti-IgE to produce anaphylactic degranulation or examined in buffer-incubated controls. Histamine release was measured in replicate samples. Control, isolated HSMC studied by electron microscopy were well preserved and fully granulated. Although all granule patterns reported for human mast cells were found, crystal granules were the most prevalent, as is true for HSMC in situ. Individual mast cells containing both crystal and scroll granules occurred. Lipid bodies were rare, as in HSMC in situ. Control, isolated mast cells did not express granule changes associated with either piecemeal degranulation or recovery during wound healing in situ; nor were morphologic changes of anaphylactic degranulation present. Spontaneous histamine release was 0% in control samples. Anaphylactic degranulation of isolated HSMC was accompanied by 24% maximum histamine release and characteristically showed extrusion of altered, membrane-free granules through multiple pores in the plasma membrane to the exterior of the cell. Other morphologic aspects of anaphylactic degranulation, as expressed in isolated human lung mast cells, were also present. These events included granule swelling, fusion, alteration of matrix contents, degranulation channel formation, pore formation, and shedding of granules, membranes, and surface processes. The ultrastructural morphology of isolated HSMC and their IgE-mediated degranulation shows some differences from similar studies of isolated human lung mast cells and of human lung and gut mast cells in biopsy samples. These differences include crystal granules as the predominant granule pattern, minor numbers of lipid bodies, and extrusion of granules during anaphylactic degranulation as characteristic for HSMC. By contrast, isolated human lung and gut mast cells have more scroll granules and particle granules, respectively, and more lipid bodies. In isolated human lung mast cells, anaphylactic degranulation is almost exclusively an intracellular fusion event characterized by the formation of complex degranulation channels within which altered granule matrix materials solubilize. In addition to morphologic differences between mast cells of skin, lung, or gut origin, functional differences have also been reported among mast cells of these organs. The ultrastructural morphology of isolated HSMC is identical to that of skin mast cells in biopsy samples, thereby validating the usefulness of this new source of HSMC for correlative functional and morphologic studies.