Detailed methods for: (1) 131I iodination of human growth hormone (HGH); (2) purification of labeled HGH (HGH-131I; and (3) radioimmunoassay of HGH are given. Bio-Gel filtration is introduced as a rapid and reproducible method for purifying HGH-131I which has been obtained by a modification of the method of Greenwood et al. (1). Highly purified HGH-131I with a bindability of 96% or more is usually achieved.