Nonsuppressible insulin-like activity (NSILA), as measured in the presence of insulin specific antiserum (GPAS) in the isolated fat cell and rat hemidiaphragm bio-assays, was demonstrated in the serum and in acid-ethanol extracts of tissues of the male Wistar rat. By this technique, no clear single organ site of synthesis or storage of this material was discerned. When rat serum was chromatographed on Dowex 50 × 8 resin and subjected to zone electrophoresis on a Pevikon block, its behavior was identical with that of human serum. Further studies were carried out on a single selected tissue extract, liver, employing: Pevikon electrophoresis, Sephadex G-100 gel filtration, biologic activity on muscle and adipose tissue, heating and thiol inactivation. By these criteria, NSILA, extracted from rat liver, has a molecular weight approximating 50,000, migrates in the gamma and beta globulin region on electrophoresis and in all other ways tested is quite analogous in its behavior to human serum NSILA. Isolated rat...