The inflammatory infiltrate within human sural nerve, biopsied from six patients with active chronic inflammatory demyelinating neuropathy (CIDP) was studied for T lymphocyte subsets and Class II antigen (Ia)-expressing cells. Immunohistochemical staining with mouse monoclonal antibodies, acid phosphatase staining, and electron microscopy were used to provide an alternative assessment of macrophage and other mononuclear cell numbers. In normal control nerves Class II antigen was present upon endothelial cells, very occasional mononuclear cells and sparsely within the perineurium. In CIDP nerve dense Class II antigen staining was prominent within nerve fascicles, in capillary endothelial cells and within the perineurium. T lymphocytes of suppressor and helper type were present in small numbers only. Moderate numbers of macrophage-monocytes were found in the patients within nerve fascicles but these cells accounted for only part of the dense Ia staining. Since two nerves with hypertrophic changes, Schwann cells forming 'onion bulbs', were clearly Ia positive, the dense and widespread staining in all nerves studied is best explained by Ia antigen expression upon mononuclear and some Schwann cells.