The immunogenicities of six recombinant human growth hormone (rhGH) preparations, from KABI (A rhGH191 and B rhGH192), Eli Lilly (C), Nordisk (D), Sanofi (E) and Serono (F), used to treat 260 GH-deficient children, have been compared using a common specific and sensitive procedure for antibody determination. For this purpose we developed two immunoassays: a competitive liquid radioimmunoassay using 125I-rhGH, and an immunometric solid enzymoimmunoassay in which the rhGHs were immobilized. Blood samples were collected from the GH-deficient children before treatment and after 3, 6, 9, 12, 18 and 24 months of therapy. Human GH antibodies were detected in children treated with 3 of the 6 rhGH preparations. Seven percent of the patients treated with hormone A, 14% with hormone B and 22% with hormone C formed antibodies against the respective rhGH. Differences in capacity and affinity of the hGH antibodies were observed between these anti-GH-positive groups. They could be divided into 2 groups according to their immunopotency. One group (7, 14 and 6% of the patients treated with hormones A, B and C, respectively) developed anti-hGH antibodies with very low binding capacities (30–100 fmol/ml). The other group (16% of the patients treated with hormone C) developed IgG-type antibodies to hGH with higher binding capacities (200–1,200 fmol/ml) and a measurable binding affinity (Ka = 108M-1). These hGH antibodies partially inhibited the binding of labeled GH to its specific liver membrane receptor. However, because of their low titer, they did not inhibit growth in the treated children. The antibody response profiles after 24–30 months of rhGH C therapy indicated a progressive increase in hGH antibody levels up to 6 or 9 months of treatment, followed by a decrease to undetectable levels at 30 months. These results show that currently available rhGH preparations have attained a great degree of purity and have a very low immunogenicity. Sensitive techniques, such as those used in the present study, may help to detect immunogenic differences between recombinant hormones and should be used to assess the quality of new preparations.