Discrepancies between GH measurements and growth rate of children have complicated diagnosis in a variety of clinical conditions. The competition of GH-BP with the GH-receptor towards GH-receptor binding can have a role in these discrepancies. A mathematical model was developed for appraising the availability of GH for receptor binding from measurements of serum GH by RIA and serum GH binding protein (BP) by a binding assay. Eighteen patients with high GH-BP (obesity), normal GH-BP (normal control) or low GH-BP (children, anorexia nervosa or cirrhosis of the liver) were the subjects of this study. Sera of patients with high, normal or low GH-BP levels were analyzed for their competition with [125I]hGH binding to rabbit liver membranes. Serum GH was measured by a commercial polyclonal RIA. Serum GH-BP was measured by a binding assay with dextran-coated charcoal separation. Receptor availability for GH was assessed by displacing of [125I]hGH from rabbit liver membranes. The decline in receptor availability for each hGH value, caused by GH-BP competition with the receptor, was calculated by subtraction of the percent displacement in the absence of GH-BP from the percent displacement in the presence of a given GH-BP value. The results were analyzed statistically to give a series of polynomes. These enabled the calculation of an activity factor for serum RIA GH levels, that should predict the receptor availability of each GH level, according to the concomitant GH-BP level.(ABSTRACT TRUNCATED AT 250 WORDS)