The adult normal human spiral ganglion (SG) was analyzed with regard to ultrastructure and immunohistochemistry. The cytoskeleton of the SG cells was found to comprise F-actin, intermediate filaments (IFs) and microtubules (MTs). The IF subgroups (cytokeratins, Cks; neurofilaments, NFs, vimentin, glial fibrillary acidic proteins, GFAP; desmin) displayed characteristic staining patterns. Ck No. 8 was found in all SG cells, whereas vimentin was lacking. GFAP stained only a small subpopulation of SG cells (type 2?). The light (68 kD) and medium-sized chains of NFs occurred in all SG cells and axons, whereas the 200-kD NF subunit was only found in the axonal hillock of (type 2?) SG cells, but in no other part of the cytoplasm, and regionally in nerve fibres. MAP-1 and MAP-2 occurred in all SG cells but only MAP-1 was found in the nerve fibres. The calcium-binding protein synaptophysin (SY) was expressed only in SG cells, in contrast to the S-100 which occurred more generally in the labyrinth. The neuropeptides VIP and substance P were identified in all SG cells, in contrast to NPY which was expressed in a small subpopulation of SG cell (type 2?). Staining for neuron-specific enolase (NSE) identified most (type 1?) but not all SG cells. The cell surface glycoprotein Thy-1 was expressed in SG cells in a way similar to that described for neurons in the CNS. The SG cells express a high degree of cytoskeletal complexity, allowing one to distinguish between type 1 and type 2 cells. The cell bodies and their adjacent nerve fibres show characteristic features of calcium-binding proteins, surface membrane glycoproteins, NSE and neuropeptides but the basic pattern is still similar to neurons in the CNS.