Male BALB/c mice that received prophylactic iv treatment with pyran had significantly enhanced splenomegaly, an increased number of splenic foci induced by the spleen focus forming virus (SFFV) in the Friend murine leukemia virus (F-MuLV) complex, and a slightly decreased mean survival time as compared with untreated controls infected with F-MuLV. A corresponding increase in the lymphatic leukemia virus component of the F-MuLV complex was not observed, which suggests that the enhancement of the disease was due primarily to a selective increase in the SFFV component of the F-MuLV complex. That the enhancement was related to an increased number of target cells for SFFV was substantiated by data concerning erythropoiesis in iv pyran-treated animals. Increases in splenic hematocrits and in uptake of 59Fe in the spleens of animals treated iv with pyran provided quantitative evidence for the histologic finding of increased erythroid precursors in the spleens.