Autoantibodies directed against proteins of the adrenal cortex and the liver were studied in 88 subjects of Sardinian descent, namely six patients with autoimmune polyendocrine syndrome type 1 (APS1), 22 relatives of APS1 patients, 40 controls with other autoimmune diseases, and 20 healthy controls. Indirect immunofluorescence, using tissue sections of the adrenal cortex, revealed a cytoplasmatic staining pattern in 4 of 6 patients with APS1. Western blotting with adrenal mitochondria identified autoantigens of 54 kDa and 57 kDa, Western blotting with placental mitochondria revealed a 54-kDa autoantigen. The 54-kDa protein was recognized by 4 of 6 patients with APS1 both in placental and adrenal tissue, whereas the 57-kDa protein was detected only by one serum. Using recombinant preparations of cytochrome P450 proteins, the autoantigens were identified as P450 scc and P450 c17. One of six APS1 patients suffered from chronic hepatitis. In this patient, immunofluorescence revealed a centrolobular liver and a proximal renal tubule staining pattern. Western blots using microsomal preparations of human liver revealed a protein band of 52 kDa. The autoantigen was identified as cytochrome P450 1A2 by use of recombinant protein preparations. P450 1A2 represents the first hepatic autoantigen reported in APS1. P450 1A2 usually is not detected by sera of patients with isolated autoimmune liver disease and might be a hepatic marker autoantigen for patients with APS1.