The development of anti-human growth hormone (anti-hGH) and anti-host-cell protein antibodies to recombinant hGH (rhGH) of mammalian cell origin was determined in 395 children (304 GH deficiency; 91 Turner syndrome) undergoing long-term treatment (up to 54 months) for growth disorders. In all patients, blood samples were obtained prior to and every 2-3 months during treatment, and analyzed at a central laboratory for anti-hGH antibodies by RIA and antibodies to host-cell antigens by ELISA. During the first 24 months of treatment, 9 (3%) of the 304 patients with GH deficiency developed antibodies to rhGH for longer than 3 months. However, persistent antibodies were seen in only 2 patients, both of whom had proven hGH-N gene defects. In the remaining 7 (2%) anti-rhGH-antibody-positive patients, antibody concentrations showed a tendency to increase for 3-12 months, irrespective of the time of onset of measurable concentrations, and declined thereafter. In these patients, binding capacities were between 0.01 and 0.1 mg/l, and binding affinities were between 7 × 10<sup>8</sup> and 8 × 10<sup>9</sup>1/mol. Height velocity was unaffected in these children. None of the 91 patients with Turner syndrome developed persistent anti-hGH antibodies. Further, no child developed antibodies to host-cell antigens during treatment with rhGH of mammalian cell origin.