Spleen Focus Formation by Polycythemic Strains of Friend Leukemia Virus

Several strains of Friend virus were tested for their ability to initiate focus formation in the spleens of susceptible mice. Strains which induce polycythemia had high focus-forming activity, while strains which induce anemia had little or no focus-forming activity. A positive correlation was also demonstrated between virus strains with focus-forming activity and those which initiate the resumption of erythropoiesis in plethorized mice. It is concluded that most Friend virus preparations contain at least 2 viruses: one which has spleen focus-forming activity (SFFV) and which probably is responsible for inducing polycythemia when present in high titers, and another which, when active in the presence of little or no SFFV, induces anemia. The role of either virus in initiating leukemia remains to be determined.