17 children with growth retardation (12 with idiopathic hypopituitary dwarfism, 2 with craniopharyngioma and 3 constitutionally short) were studied for three days following a single intramuscular injection of human growth hormone. Somatomedin activity was bioassayed using both sulphate incorporation into chick embryo cartilage and thymidine uptake by human lectin-activated lymphocytes. In hypopituitary patients it showed a significant response, maximal 24 hours after the injection, and significantly correlated for the two bioassays. The aminoacid content of the incubation medium used for thymidine bioassay appeared as an important factor: both glutamine and nonessential aminoacids are required to obtain significant stimulation by low serum concentrations, thus increasing the sensitivity of the assay but reducing the differences between normal and hypopituitary sera. Transferrin levels in serum were significantly lower in hypopituitary dwarfs. They did not rise in the three days following hGH. Aminoacid levels were lower in idiopathic GH deficient patients than in other groups, and did not show short term increase in the fasting samples collected after hGH administration.