Human thyroid-stimulating hormone (H-TSH) has been prepared from a side fraction, obtained during the isolation of human growth hormone, which contained thyroid-stimulating and gonadotrophic activity. After a preliminary equilibration step on Sephadex G-50, the thyrotrophin activity was adsorbed on carboxymethyl-cellulose (CM-C). Seventy-two per cent of the proteins were unadsorbed. The H-TSH was eluted and placed on a Sephadex G-100 column. These 2 steps gave a 35-fold purification. An additional 3-fold purification was achieved on diethylaminoethyl- cellulose (DEAE-C). The final potency of the preparation was 20 IU/mg. The behavior of the H-TSH, during disc electrophoresis and on the ion exchangers, DEAE-C and CM-C, suggests that the H-TSH is more acidic in nature than the bovine hormone. On G-100 the human and bovine hormone occupy the same position, from which it may be concluded that they have approximately the same molecular weight.