High-Yield Isolation of Mouse Mammary Tumor Virus

Rate-zonal centrifugation in fhe zonal ultracentrifuge with subsequent isopycnic banding in a Ficoll in heavy-water gradient was used on milk from mice with a high incidence of mammary cancer. This method provided a rapid means for isolating large quantities of mammary tumor virus (MTV) in a relatively high state of purity. The physical properties of the virus in these systems were investigated. The isopycnic banding density was 1.156 g/ml, and the corrected sedimentation coefficient (S20,w) was between 900 and 1000 for adequately dispersed B particles. Virions, apparently incompletely dispersed, had a broad range of sedimentation coefficients, most particles sedimenting with an S20,w of about 1400. When examined in the electron microscope, the isolated particles appeared nearly free of extraneous material, and frozen and thawed samples were tumorigenic at a dilution of 10−4. By a recently developed MTV hemagglutination immunoassay, virus preparations were shown to be free of interfering substances and consistently gave high liters.