The alpha 7 acetylcholine receptor (AChR) has been linked with the onset of psychotic symptoms and we hypothesized therefore that it might also be an autoimmune target. Here, we describe a new radioimmunoassay (RIA) using iodine 125-labelled alpha-bungarotoxin and membrane extract from transfected HEK293 cells expressing human alpha 7 AChR. This RIA was used to analyze sera pertaining to a cohort of 711 subjects, comprising 368 patients diagnosed with schizophrenia spectrum disorders, 140 with bipolar disorder, 58 individuals diagnosed of other mental disorders, and 118 healthy comparison subjects. We identified one patient whose serum tested positive although with very low levels (0.2 nM) for alpha 7 AChR-specific antibodies by RIA. Three out of 711 sera contained antibodies against iodine 125-labelled alpha-bungarotoxin, because they precipitated with it in the absence of alpha 7 AChR. This first evidence suggests that autoantibodies against alpha 7 AChR are absent or very rare in these clinical groups.