The TSH (thyrotrophic hormone) induced stimulation of the rate of incorporation of radioactive phosphate into the phosphatides of the thyroid was studied in vitro. Since neither thyroid hormones nor agents blocking iodine metabolism altered the 32P uptake, a direct effect of TSH on thyrophosphatides was assumed. Paper and column chromatographic experiments on phosphatides isolated from beef thyroid slices after incubation with TSH demonstrated that the 32P incorporation occurs into lecithins, cephalins and an inositol-phosphatide. The action of TSH on thyrophosphatides did not appear to be restricted to one cell fraction, while the TSH controlled phosphatides appeared to belong to the so-called bound class. A structural function of the thyro-phosphatides is discussed.