Fourteen alveolar soft part sarcomas were studied using electron microscopic analysis and a battery of immunocytochemical procedures in order to critically assess the extent to which these methods could explain the histogenesis of the tumor. All the tumors were stained with antibodies against keratin, vimentin, S-100 protein, Leu-7, neuron-specific enolase, desmin, muscle-specific actin, myoglobin, smooth muscle myosin, and fast and slow myosins. Eight tumors stained for vimentin, eight for desmin, eight for neuron-specific enolase, two for muscle-specific actin, and one for fast myosin. No reactivity was detected with the other antibodies. Electron microscopic study showed a consistent pattern of fine structure and clarified the mode of formation of the distinctive cytoplasmic crystals which were detected in all 14 tumors. Intermediate filaments were sparse and specific myofilaments were not detected. Alveolar soft part sarcoma has a myogenic phenotype and the cells show skeletal muscle differentiation but do not closely resemble any known prototypic normal cell.