Patients with paraneoplastic encephalomyelitis(subacute sensory neuronopathy) (PEM/SSN), most commonly associated with small-cell lung cancer (SCLC), frequently harbor Hu antibodies, which are usually detected by indirect immunohistochemistry or immunoblot. We developed a new radioimmunobinding assay to detect Hu antibodies based on in vitro transcribed and translated (ITT) HuD. High levels of Hu antibodies were detected in all seven PEM/SSN patients tested, but not in any of 15 patients with other paraneoplastic syndromes, 20 patients with Sjögren's syndrome, 20 patients with miscellaneous tumors and 99 of 100 blood donors. One of the blood donors had low levels of Hu antibodies. The radiobinding assay detected Hu antibodies in 45 of 191 (24%) patients with SCLC, in comparison with immunofluorescence and immuno dot blot, where only 34 of 191 (18%) were detected. All patients with Hu antibodies detected by immunofluorescence and immuno dot-blot were also positive by the radioimmunobinding assay. The results demonstrate that this assay is very specific and sensitive for the detection of Hu antibodies.