Abstract An enzyme-linked immunosorbent assay (ELISA) has been developed for detection of antimitochondrial antibodies (AMA) in patients' sera. Results are expressed as a mitochondrial binding index, calculated from binding of patients, antibodies to rabbit liver mitochondrial fraction and total liver homogenate. The incidence of AMA detectable by this method in 22 sera from patients with primary biliary cirrhosis was identical with that obtained by conventional immunofluorescence testing (86%), and there was a highly significant correlation between titres determined by the two methods ( r=0.828; P < 0.01) although ELISA titres were far higher. AMA were also detectable by ELISA in sera from 9 22 patients which chronic active hepatitis, 1 38 patients with other liver diseases, 3 16 patients with syphilis, 2 16 patients with non-hepatic autoimmune disease and 0 29 normal blood donors; of those, only 2 chronic active hepatitis sera were positive for AMA by immunofluorescence. The ELISA titre was significantly correlated with the mitochondrial binding index determined at a serm dilution of 1:200 ( r= 0.793; P < 0.01), allowing an estimate of antibody titre to be made from ELISA screening at this single serum dilution. This assay, which is simple and reproducible, may be of value as an objective method of Screening for AMA.