A simple method is described for the rapid separation of cells [such as Chinese hamster V79 lung fibroblasts], containing radioactive material, from their suspending medium. Cells, in aqueous medium, are layered on fetal bovine serum in microfuge tubes. The tubes are immediately spun for 1 min, frozen in ethanol-dry-ice, and their tips sliced and assayed. This method is both faster and more convenient than currently employed techniques.