Abstract A direct binding assay for the detection of mitochondrial autoantibodies (AMA) is described. The method has been standardised to use as antigen 1 mg/ml submitochondrial particles which are absorbed on to the surface of LINBRO S-MVC-96 microtitration plates as the solid phase. 125I-labelled antihuman immunoglobulin is used as the second antibody. The sensitivity achieved is greatly increased compared with immunofluorescence and complement fixation methods, with good discrimination between AMA negative and positive sera. The assay is rapid and has the advantages of a primary binding method. Two variatios of the method are described. The first is reported as a ‘binding index’ which is calculated from the binding shown by an unknown serum at 2 dilutions, and would be suitable as a screening procedure. Binding curves of doubling dilutions of a test serum give, in addition to the binding index, a titre which is defined as that dilution at which binding returns to the level of a normal serum. This is a longer procedure and more suitable as a research method.