The GH dependency of somatomedin (SM) peptide content in plasma was assessed using the placental membrane radioreceptor assay (RRA) to measure whole and acid-chromatographed plasma using both [125I]SM A and [125I]SM C. Plasmas from 10 normal children and adults, 11 GH-deficient children before and after therapy with 0.1 U/kg GH im daily for 5 days, and 10 acromegalic patients were assayed as well as a standard plasma pool from 20 healthy adult males. The results of these assays revealed that the SM content of plasma is GH dependent and that assay of whole and acid-chromatographed plasma with the [125I]SM A RRA and assay of acid-chromatographed plasma with the [125I]SM C RRA give comparable measurements of SM peptide content. There was a positive correlation of the [125I]SM A RRAs of whole and acid-chromatographed plasma from each patient (r = 0.87; n = 41; P much less than 0.001). An even stronger positive correlation was observed when the [125I]SM A and [125I]SM C RRAs of acid-chromatographed plasma were compared (r = 0.94; n = 41; P much less than 0.001). Furthermore, it was demonstrated that precise measurement of SM peptide content in plasmas from subjects with abnormal GH states and from normal subjects is enhanced by acid chromatography of the plasma before assay in either the [125I]SM A or [125I]SM C RRA.