Sera from 88 patients with progressive systemic sclerosis were examined for precipitating mitochondrial antibodies using sonicated rat liver mitochondrial fraction as an antigen source in immunodiffusion. Precipitin lines indicating the presence of anti-mitochondrial antibodies (AMA) in 22 patients were detected. Only six of 22 sera had, additionally, precipitating antibodies to nuclear antigens. Standardized reference sera containing antibodies to mitochondrial antigens (M-A, M-B and M-C systems) were used to further characterize the type of mitochondrial antibodies. M-B antibody was most commonly detected (72.7%) either alone (eight patients) or in combination (eight patients) with M-A and M-C antibodies. M-A antibody was found in 12 patients (54.5%) and M-C antibody was present in three. The antigen related to M-B is DNAase and trypsin sensitive, in contrast to the resistant M-A antigen. AMA were detected in 21 of 22 patients by indirect immunofluorescence. When solid phase ELISA was used to detect AMA using mitochondrial fraction as antigen, a significant difference (P less than 0.005) was noted between sera with and without precipitating mitochondrial antibody. The antibody was frequently present in patients with progressive systemic sclerosis detected 2 or more years earlier (P less than 0.01). Three patients were found to have primary biliary cirrhosis and others had pruritus, hepatomegaly or abnormal liver function tests. The implication of the findings is discussed.