Rats were given 1.25 uc/g body weight of radioactive sulfate intravenously. Peritoneal cells were collected and separated by centrifugation through sucrose solution into a mast cell fraction and an "other" peritoneal cell fraction. The separated cells were collected on Millipore filters (Millipore Filter Corp., Watertown, Mass.) and radioactive counts measured with a Geiger-Mailer tube. Radioactivity of mast cells reached a maximum in 48 hours and did not diminish appreciably in the following 5 days. Other peritoneal cells attained their maximum radioactivity in 4 hours with a subsequent rapid falloff. Possible applications of this new method for direct measurement of cell radioactivity are suggested.