A modified plasma radioimmunoassay has been developed that uses a cellulose slurry to separate bound from free insulin. This material, which is commercially available, shelf stable, and simple to prepare, makes it possible to run the entire method in test tubes and to use a low specific activity insulin. Details concerning conditions and materials that determined sensitivity, accuracy, and precision are reported. The method is sensitive to about 10 μU/ml. of plasma and has a reproducibility of approximately ± 15 per cent. This method is proposed for use by those who desire to measure rises in plasma insulin by a simple, rapid and reasonably accurate technic.