A simple bioassay is described for insulin-like activity (ILA) which is based on the incorporation of (3H)glucose into the lipids of isolated rat fat cells. Total lipids were extracted by adding toluene-based scintillation fluid directly to the incubation flasks, and the radioactivity in the lipids measured by liquid scintillation counting. Insulin dose-response curves with (2-3H)-, (3-3H) and (6-3H)glucose were compared and those obtained with (2-3H)glucose and (3-3H)glucose were found suitable for routine use.