Intranuclear inclusions of the testicular Leydig cells were described in an autopsy case of synovial sarcoma. The inclusions were histologically identical to the intranuclear inclusions of Cowdry's type A. Histochemical analysis disclosed that they were mainly composed of protein. Inclusions were tentatively classified into 3 groups on the basis of their ultrastructural features. Type 1 inclusions consisted of irregular grouping of electron dense, floccular material. They were usually situated centrally within the nuclei, and there were prominent margination of the chromatin granules on the nuclear envelopes. Inclusions with mixed composition of fine filaments and floccular material, disignated as type 2, were evenly distributed in the karyoplasm. Type 3 inclusions consisted of numerous clusters of fibrillar structures randomly oriented to each other. The fibrins were composed of parallel rows of 4 or 5 dense fine filaments. The incidence of the type 3 inclusions is lower than that of the other two. In no instance were intranuclear inclusions and Reinke crystals encountered in the same cell.