The RNAs of several avian tumor virus recombinants that had inherited their focus-forming ability from a sarcoma virus and their host range marker from a leukosis virus were investigated. Electrophoretic analyses showed that the cloned sarcoma virus recombinants contained only size class a RNA, although they had acquired a marker that resided on class b RNA in the leukosis virus parent. Class a RNA of different recombinant clones, derived from the same pair of parental viruses and selected for the same biological markers, differed slightly in electrophoretic mobility from each other and from the parental sarcoma virus. They were also found to have different fingerprints of RNase T1-resistant oligonucleotides. The average complexity of the 60-70S RNA prepared from Prague Rous sarcoma virus of subgroup B was estimated to be 3.5 x 10(6) daltons from the size of 20 RNase T1-resistant oligonucleotides, which represented 3.9% of the RNA and that of a recombinant to be 3.3 x 10(6) daltons from 23 oligonucleotides, which represented 4.7% of the RNA. This result suggests that the genome of wild-type and of recombinant RNA tumor viruses is polyploid. The sum of these observations led us to propose that recombination among avian tumor viruses occurred by crossing-over between homologous pieces of nucleic acid.