SUMMARYReducing equivalents derived from the tissue re‐oxidation of NADPH (NADPH‐diaphorase) have been implicated in the peroxidation that is involved in the organification of iodine in the production of thyroid hormones. Immunoglobulin (Ig) fractions from patients with thyroid diseases and from normal controls, in a standard dose of 125 μg/ml and 0±3 μ/ml thyrotrophin (TSH) were incubated with segments of guinea‐pig thyroid gland maintainedin vitro.A quantitative cytochemical study was made on how these fractions influenced the enzyme activity. A good correlation was found between the ability of such Ig fractions to stimulate the NADPH‐diaphorase activity and (1) the degree of hyperthyroidism in the patients and (2) the amount of T3 secreted by the thyroid segmen