Autoantibodies to neurons and to the cytoskeleton were demonstrated in the serum of a patient with small cell carcinoma of the lung (SCCL) and paraneoplastic sensory neuropathy. The serum reacted by immunofluorescence with the nuclei of neurons, and with cytochalasin B-sensitive "stress fibres" of cultured cells. The serum also reacted by immunofluorescence with the nuclei of some cultured cell lines. Immunoblotting experiments with brain tissue, with SCCL, HeLa and other cultured cells showed that the serum reacted with 1-4 bands of 35-39 kDa apparent m.w. Two dimensional immunoblotting showed that these molecules had a neutral pI. Antibodies, affinity-purified by elution from the 35-39 kDa bands, gave staining of the nuclei of neurons and of cultured cells and immunoblotted the same 35-39 kDa antigens. These observations show that the anti-neuronal autoantibody reacts not only with neurons and with SCCL but also with some cultured cell lines. Molecular mimicry has been invoked as the basis for the reactivity of this autoantibody with exposed epitopes on SCCL and on neurons.