Introduction. Studies on recombinants between two closely related viruses have proved a useful approach for biochemical and antigenic analyses of virus genomes. For example, recombinants between adenovirus type 5 and the non-defective adenovirus 2-SV 40 hybrid, Ad2+ND1, have been used to map a number of gene functions and to correlate the adenovirus genetic and physical maps (Mautner et al. 1975; Williams et al. 1975). Analysis of recombinants between different influenza viruses (Palese, 1977) or reoviruses (Mustoe et al. 1978) has permitted correlation of polypeptides with specific RNA segments while T3 × T7 recombinants have been used to map most of the promoter sites of the major RNA species transcribed by T3 and T7 RNA polymerases on T3 and T7 DNA (Beier et al. 1977). Such analyses can be extended to any pair of genomes that recombine with each other.