A quantitative immunoassay of human growth hormone has been developed from a modification of the micro-Ouchterlony double diffusion gel technique. The method offers the advantages of speed and simplicity and differs from other immunologic techniques in that it can separate and quantitate a single antigen-antibody reaction. The procedure has been used to assay the growth hormone activity of human pituitary extracts. Data are presented to show the specificity and reproducibility of this method of assay. The results obtained with the immunoassay were compared with those obtained by the rat tibia bio-assay, and a highly significant correlation was found between the 2 methods. This immunoassay does not have the sensitivity necessary for assaying growth hormone in unconcentrated human serum, but, by providing a rapid and reliable estimate of the growth hormone potency of human pituitary fractions, it has facilitated work on purification of growth hormone: