Fractions of human pituitary prepared by (NH 4) 2 S0 4 precipitation and chromatography on an" Amberlite" column were tested for growth hormone by their effect on width of tibial epiphyses in hypophysectomised rats. Prolactin activity was detected and estimated semiquantitatively by the localized response of mammary glands of pseudopregnant rabbits. One fraction studied was found to have high growth hormone activity, and lactogenic response to it was similar to that to purified sheep prolactin at similar dose levels. The fraction and sheep prolactin also behaved similarly during starch-gel electrophoresis, but the former had little crop-stimulating activity for pigeons. It was suggested that growth hormone and prolactin may not have separate identities in man as in sheep and ox.-JE Hammant.