Sixty-four blood samples, obtained from 25 pituitary dwarfs treated with KABIHGH preparation, were checked for serum HGH concentration by a double antibody radioimmunoassay and for antibodies against HGH by the method of propylethyleneglycol separation. Antibodies to HGH were detected in the samples whose HGH concentration was falsely estimated to be more than 5 ng/ml, but not in those whose HGH concentration was 5 ng/ml or less. The measurement of HGH concentration by a double antibody technique in the serum obtained from pituitary dwarfs 3-4 days after the last injection can be used to screen the presence of antibodies to HGH peparation used.