Nucleotide sequence specific complementary DNA's of MSV were used to show that there was an increase in the number of sarcoma specific and common sequences during a productive infection, and a loss of both types of sequences during reversion in cat S plus L minus cells. Analogous cDNA fractions were also obtained from feline sarcoma viruses. Recombinant MuLV's were shown to have gp70 constituents of eco- and xenotropic viruses and the gag region of ecotropic viruses. These viruses in pure form could cause lymphoma in mice. Gene substitution recombinants were also detected. Seroimmunoprophylaxis studies showed that the life span can be significantly prolonged in high leukemia strains of mice. Antibodies other than those directed against gp70 could also be critical in tumor rejection.