Human pituitary glands were incubated with 3H-l,4,5-leucine in Krebs-Ringer bicarbonate buffer to study the biosynthesis and secretion of human pituitary proteins generally and growth hormone specifically. After 4 and 24 hr, anti-HGH serum precipitated only 3 and 13%, respectively, of the 3H-proteins in the medium. Following gel filtration of the medium on Sephadex, a single radioactive protein peak emerged with an elution volume which was not quite coincident with the elution volume of HGH and the protein peak. FSH, LH and TSH were present in earlier fractions and ACTH in later fractions. The total amount of all 4 hormones in the media was a thousandfold less than the amount of HGH in the media. Anti-HGH serum precipitated less than 5% of the 3H-proteins in the tubes which contained HGH, yet the latter accounted for more than 80% of the proteins in these fractions. Hence, the specific activity of the remaining proteins present in these fractions was very much greater than that of HGH. In the tissue, anti-HGH serum precipitated 3H-proteins of large molecular weight (mol wt greater than 100,000) and also 3H-proteins which have the same elution volume as the HGH standard. Upon acrylamide gel electrophoresis the Rf of the major radioactive protein was 0.55 compared to an Rf of 0.6 for HGH and FSH. When rat anterior pituitary glands are incubated in a similar manner, the principal radioactive protein released into the medium is rat prolactin. We suggest that the principal radioactive protein secreted into the medium in vitro by human pituitary glands is also human prolactin, but the chemical amount present is extremely small relative to HGH.