ICANCER RESEARCH 33, 5—10,January 1973] SUMMARY Neither the infectivity nor the morphology of the mouse mammary tumor virus is appreciably affected by abrupt dilutions from strong solutions of cesium chloride, sodium chloride, or sodium iothalamate. In the purification

Neither the infectivity nor the morphology of the mouse mammary tumor virus is appreciably affected by abrupt dilutions from strong solutions of cesium chloride, sodium chloride, or sodium iothalamate. In the purification of this virus by density gradient centrifugation, it is not necessary to use solutions of low osmotic activity. In sucrose, sodium iothalamate, potassium tartrate, or citrate, the buoyant density of the main virion band was found to be 1.18 g/ml; in cesium chloride, it was approximately 1.22 g/ml. A correlation was found between the apparent integrity of particles and their infectivity in bands recovered from various gradients. The infectivity titer of the main virion band varied with the gradient medium used; but sucrose, cesium chloride, Ficoll, potassium tartrate, or citrate and sodium iothalamate all were satisfactory, sucrose probably being preferred. The distribution and appearance of bands varied greatly from one gradient to another. Gradients of neutralized sodium phosphotungstate destroyed both the infectivity and the morphological integrity of the virions.