These studies domonstrate that Friend Virus produces in Ha/ICR Swiss and DBA/1 mice very soon after infection, a marked increase in selective uptake of Fe59 by the spleen. This selective uptake is not found in other tissues in Friend Virus Disease. Friend Virus Disease characteristically produces a malignant proliferation of splenic reticulum cells, but is further characterized by a progressive elevation of hematocrit and rbc count which is related to splenic involvement in the disease. The DBA/1 strain is shown to be susceptible to Friend Virus to the same magnitude as HA/ICR Swiss mice.