By employing a new competitive proteinbindingassay described in the previous paper, serum levels of the somatomedin designated as nonsuppressible insulin-like activity (NSILA) have been measured in normal individuals and patients with disorders of GH secretion, both under basal and altered conditions of carbohydrate metabolism. In addition, serum NSILA levels have been determined in several species of laboratory animals (mouse, rat, rabbit, dog, and sheep). Evidence that serum NSILA is GH-dependent is provided by the finding that the mean (± SEM) level in 11 acromegalics (1136 ± 60/μU/ml) is significantly higher (P < 0.001) and the mean level in 8 hypopituitary patients (408 ± 30 μU/ml) is significantly lower (P < 0.001) than that in 14 normal subjects (836 ± 21 μU/ml).Also, after the administration of human GH to three hypopituitary patients, the mean NSILA level rose from a baseline of 409 ± 108 /μU/ml to 628 ± 48 μU/ml. Serum NSILA levels were not significantly changed either by insulin-induced hypoglycemia or glucose ingestion, and no diurnal variation was found. NSILA is significantly lower (P < 0.01) in the umbilical cord sera of neonates(595 ± 85 μU/ml) than in paired maternal samples (942 ± 65 μU/ml). Serum NSILA mainly appears to be bound to a large molecular weight carrier protein, with only 0–0.4% circulating in the “free” form. The competitive protein-binding assay also demonstrates extensive crossreactivity of NSILA in various species of animals with NSILA and its carrier protein derived from pooled human plasma.