The presence of serum immunoglobulins (Ig) blocking ACTH-induced adrenal DNA synthesis and/or cortisol production was studied in 25 patients with idiopathic Addison's disease. For this purpose guinea pig adrenal segments kept in organ culture were exposed to ACTH and graded concentrations of patient IgG. After a 5-h culture period the cortisol present in the culture fluid was measured by RIA, and DNA synthesis in the adrenal cells was measured using Feulgen densitometry on frozen sections of the cultured adrenal segments. Addition of ACTH alone in concentrations of 0.1-10 pmol/L to the culture system stimulated in vitro cortisol secretion; the maximal stimulation was 63 +/- 35% (+/- SD; n = 5) at a concentration 0.1 pmol/L. ACTH also increased (in concentrations of 1 fmol/L to 1 pmol/L) the percentage of fasciculata cells in S-phase from 0-4% (nonstimulated) to 5-12%. IgG preparations from all but 2 of the 25 patients with idiopathic Addison's disease blocked these in vitro ACTH-induced adrenal responses in a dose-dependent fashion. IgG from 2 patients with tuberculous adrenalitis, 1 patient with secondary adrenal insufficiency, and 7 normal subjects had no blocking activity. Among 5 non-Addisonian autoimmune endocrinopathy patients who had adrenal cytoplasmic autoantibodies, 4 had no ACTH-blocking IgGs. Two of 9 patients with miscellaneous adrenal disorders (Cushing's disease, pigmented adrenal micronodular dysplasia, and adrenal nodules) had ACTH-blocking activity. These results demonstrate the existence of IgGs blocking the in vitro effects of ACTH and suggest their involvement in the pathogenesis of idiopathic Addison's disease.