The mononuclear cell infiltrates in autopsy material from brains and spinal cords of six patients with multiple sclerosis (MS) were characterized in situ. Receptors for sheep erythrocytes and anti-T lymphocyte serum were used to identify T lymphocytes. Receptors for the activated third components of complement and for the Fc-portion of IgG (Fc gamma receptors) were used as markers for macrophages and lymphocytes. In acute and recent MS lesions, perivascular infiltrates of mononuclear cells were commonly found. Most cells were T lymphocytes. Some B lymphocytes were also demonstrated. The demyelinating lesions contained large numbers of lipid macrophages expressing Fc gamma receptors. The lipid macrophages are probably derived from microglial cells. The chronic, demyelinated lesions contain a few mononuclear cells, mainly B lymphocytes and plasma cells.