S-100 beta-positive T-lymphocytes in human peripheral blood were shown to be definitely T-cells containing the S-100 beta subunit in their cytoplasm by double immunostaining using immunoferritin labeling for CD8 antigen and immunoperoxidase labeling for cytoplasmic S-100 beta subunit at the ultrastructural level. These results ultrastructurally confirmed the presence of double markers of S-100 beta-positive T-lymphocytes seen by light microscopic double immunostaining. Furthermore, this double immunostaining method for electron microscopy is applicable to other kinds of cells for the simultaneous detection of two different antigens present both on the cell surface and in the cytoplasm.