IN 1960, Sutherland et al.¹ reported on children with familial nongoitrous cretinism born of a hypothyroid mother with Hashimoto's disease. Subsequently, Goldsmith et al.² studied this family in detail and concluded that the transplacental transfer of an unidentified thyrosuppressive factor or factors might have caused congenital thyroid suppression (partially reversible) in all six siblings. Neither the mechanism of this suppression, nor the nature of the thyrosuppressive factor, however, was clarified. In this paper, we describe two siblings with congenital transient hypothyroidism and a hypothyroid mother with Hashimoto's disease. Immunoglobulins that inhibited binding of thyroid-stimulating hormone (TSH)³ and did not contain . . .