Various physicochemical characteristics and the biological activity of two murine lymphoma viruses (S-63 and GC), originally isolated in this laboratory and previously reported as sharing common antigenic properties with human and bovine lymphomas, are defined. These viruses are stable at temperature ranges from −70 °C to +4 °C and are inactivated after incubation at +56 °C for 30 min. They withstand lyophilization and are inactivated by treatment with ethanol, acetone, ethyl ether, formalin, and phenol. Both agents are of the ribonucleic acid, double-membraned viruses, similar to myxoviruses, and are approximately 100 mμ in size.