Twenty-one children were treated for GH deficiency with authentic sequence biosynthetic human GH (somatropin), 60 micrograms/kg body weight, subcutaneously, three times weekly for 1 year or longer. The magnitude of growth response and rise in serum insulin-like growth factor I levels were similar to those expected from experience with pituitary GH and somatrem. Three patients developed serum antibodies to GH with a binding capacity greater than 0.02 mg/l, but in only one patient was the GH binding capacity greater than 1.0 mg/l and he showed no attenuation of growth response. Escherichia coli polypeptide antibodies did not rise significantly and no clinically important side-effects occurred. Somatropin is safe, effective and of low immunogenicity in the treatment of GH deficiency.