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Four cases of desmoplastic malignant melanoma were examined light microscopically and immunohistochemically. Electron microscopy was performed in three cases. Light microscopy showed that all tumors were composed of neoplastic spindle cells that infiltrated between mature collagen bundles in the reticular dermis. Some of the spindle cells had bizarre nuclei, whereas other spindle cells resembled normal fibroblasts. Melanin could not be demonstrated in any of the tumors by histochemical techniques. Electron microscopic examination of the spindle cells showed prominence of rough endoplasmic reticulum, which was dilated and filled with flocculent material and occasional collagen fibrils. The same cells contained aggregates of non-membrane-bound melanin granules and pre-melanosomes. Some cells also showed features of myofibroblasts. Immunoperoxidase staining with anti-S100 protein antibody demonstrated positivity of the spindle cells as well as of melanocytes in the basal layer of the epidermis. Scar tissue and fibroblasts did not stain. These findings show that the desmoplastic component of these malignant melanomas derives from melanocytes that have undergone adaptive fibroplasia. Therefore, in assessing depth of invasion in a malignant melanoma, measurements should include the desmoplastic areas.