Pituitary extracts from five patients with atrophic asymptomatic thyroiditis and fromfive subjects without any endocrine disease, matched for sex and age, were passed through gel filtration columns and the collected fractions were analyzed in three different radioimmunoassays (hTSH-antihTSH; hTSHβ-anti-pTSH; hLHα-anti-LHα). In the α subunit system, every profile showed a first peak corresponding to the elution position of the intact hTSH molecule and a second peak, often more important, of free α chain. In the β subunit system, ahigh molecular weight immunoreactive protein (big hTSHβ) was found in seven pituitary extracts; cross-reacting intact hTSH molecule was eluted as a second peak and free β chain was detected in all the extracts. Free α chain was generally found in greater amount than freeβ In the hTSH system, every elution pattern showed a peak of intact hTSH and another in the subunit area. In six pituitary extracts (5 thyroiditis, 1 control), a high mol wt protein was eluted (big hTSH). Its predominant presence in thyroiditis extracts seems important in view of the already described immunological heterogeneity of pituitary hTSH.