A single application of trypan blue 3 to 24 hours before or simultaneous with inoculation of the Rauscher leukemia virus (RLV) resulted in enhancement of leukemogenesis with an especially high viremia. Repeated administration of trypan blue after RLV infection resulted in reduced spleen weight. Viremia, however, was also increased compared with controls (no application of trypan blue), but in a lower rate than in mice treated with the virus plus trypan blue. Since trypan blue is known as an inhibitor of the functions of macrophages the results point at an important role of this cell type in preventing infection with this oncogenic retrovirus. In this regard there are differences in the role of macrophages in human immunodeficiency virus (HIV) infections.