The development of antibodies against HGH (Raben) has been studied in 10 children with short stature who have been treated with HGH for from 4 months up to 5 years. Significant titres of antibody against HGH, > 1 : 3100, with a low avidity to 125I-HGH were found radioimmunologically by a simple double antibody immunoassay method in three out of 10 children. In these patients a concomitant decrease in the growth rate was observed simultaneously with the appearance of antibodies against HGH. The occurrence of clinically significant titres of antibodies against HGH in all three patients coincided with the administration of one single batch (no. 5) of HGH. However, two children who initially had shown a good response to therapeutic doses of HGH developed a rapid decrease growth rate after three years of successful treatment, without any evidence of detectable antibodies against HGH in the peripheral serum. The data obtained suggest that as well as antibodies against HGH, other factors may also account for the development of resistance to the therapeutic effect of HGH.