The two main proteins p 30 and gp 70 from the Rauscher leukemia virus (RLV) have been isolated, labeled with 125I and used in radioimmune competition assays to estimate the amount of cross-reacting antigens produced during the evolution of various leukemias. The Rauscher leukemia in Balb/c mice, the radiation induced leukemia (RadLV) in C57Bl mice and two types derived therefrom by serial passages in mice (RadLV-RS) and in rats (RadLV-rat) were studied. Whereas the p 30 from RLV or RadLV (rat) viruses showed complete identity, the cross-reaction of their gp 70 proteins wasonly partial. The main findings in the tissues were the following: 1. The concentration of p 30 and gp 70 antigens increased much more during the RadLV-rat leukemia than in irradiated or RadLV (RS) treated mice. 2. In the serum, the ratio p 30/gp 70 was in general higher than one in the Rauscher leukemic mice and less than one in all types of RadLV leukemias. 3. In the spleen, RLV and RadLV (RS) infected mice had higher levels of p 30 than of gp 70. The reverse occurred in irradiated mice and in leukemic rats. Finally, the catabolic degradation of labeled RLV gp 70 was similar in normal and leukemic mice.