A congenital goiter occurring in an inbred herd of Afrikander cattle has been shown to contain an abnormal thyroglobulin which comprises most of the goiter iodine. It reacted with antibodies to highly purified bovine thyroglobulin, but formed a soluble complex. Although it is a large molecule which bears some resemblance to normal thyroglobulin, its molecular properties differ from those of normal thyroglobulin in several important respects. In density gradient ultracentrifugation, it resembled neither normal thyroblobulin nor the known thyroglobulin subunits. Although the goitrous cattle contain an iodoprotein in their serum in large amount, this is iodinated serum protein (largely iodoalbumin), which is also found as a minor component of the goiter iodoprotein. The role of the abnromal thyroglobulin in the pathogenesis of the goiter is uncertain.