Autoantibodies to anterior pituitary ACTH cells have been described in the sera from patients with Cushing's disease. We were here able to show that true ACTH cell autoantibodies do not react with the hormone itself or with Fc receptors in ACTH cells. They rather recognize a distinct pituitary cell-specific cytoplasmatic autoantigen located in a juxtanuclear position. ACTH cells from human adult pituitaries express Fc receptors producing a non-specific broad and diffuse cytoplasmic binding of normal immunoglobulins. After preparation of Fc-free F(ab)2 fractions from human immunoglobulins it could be demonstrated by immunohistochemical methods that human adult pituitary ACTH cells also contain the fetal ACTH cell autoantigen. However, Fc receptors, ACTH hormone or other proopiomelanocortin- (POMC-) derived fragments and the ACTH cell autoantigen are all located at distinct intracellular sites. ACTH cells in human fetal pituitaries were shown to lack Fc receptors. Thus, with this source of antigen the characteristic autoantibody pattern can be detected with undigested sera.