The morphology of mast cells in the skin lesion from a case of urticaria pigmentosa was observed using light and electron microscopy. Furthermore, the IgE deposition on mast cells in the lesion was studied with immunoelectron microscopy. Diaminobenzidine reaction and 4-chloro-naphthol reaction were used to detect endogenous peroxidase activity in mast cells in the lesion. Ultracytochemical techniques were applied to clarify the localization of peroxidase activity in the mast cells. Morphologically, neither blast-like cells nor intermediate cells between basophils and mast cells were found in the lesion. No mast cells in the lesion did exhibit IgE deposition on their plasma membrane. By light microscopy, granular stains of diaminobenzidine reaction and 4-chloro-naphthol reaction were observed in the cytoplasm of mast cells in the lesion. By electron microscopy, a finely stippled peroxidase staining was observed in some granules of the mast cells. Perinuclear cisterna and endoplasmic reticulum appeared devoid of enzymatic activity. These findings suggest that the infiltrating mast cells in this case were immature.