Human growth hormone (HGH) prepared from human pituitary glands by the method of Raben (1959) showed two components on agar gel electrophoresis. Fraction I corresponded to a slow alpha-globulin, and fraction II had a mobility between alpha and beta-globulin. On agar gel immuno electrophoresis using HGH and rabbit antiserum to HGH, these two components showed immunological identity. By using agar gel immunoelectrophoresis and the haemagglutination technique HGH preparations of both Raben and Li type were shown to be contaminated with human albumin. Antiserum to human growth hormone containing also antibodies against human albumin was purified by adsorption with human albumin. It is postulated that part of the difficulties encountered in the use of the haemagglutination-inhibition technique to determine growth hormone concentration in serum, is due to impurities in the human growth hormone preparations and antisera to these preparations in use.