A previously described cold-adapted attenuated virus, A/Leningrad/134/17/57 (H2N2), was further modified by 30 additional passages in chicken embryos at 25 degrees C. This virus had a distinct temperature-sensitive (ts) phenotype, grew well in chicken embryos at 25 degrees C, and failed to recombine with reference ts mutants of fowl plague virus containing ts lesions in five genes coding for non-glycosylated proteins (genes 1, 2, 5, 7, and 8). Recombination of A/Leningrad/134/47/57 with wild-type influenza virus strains A/Leningrad/322/79 (H1N1) and A/Bangkok/1/79(H3N2) yielded ts recombinants 47/25/1(H1N1) and 47/7/2 (H3N2). These recombinants inherited their ts phenotype and ability to reproduce in chicken embryos at 25 degrees C from the cold-adapted parent. Analysis of the genome composition of the recombinants obtained by recombination of the cold-adapted donor with wild-type influenza virus strains A/Leningrad/322/79(H1N1) and A/Bangkok/1/79(H3N2) showed that recombinants 47/25/1(H1N1) and 47/7/2 (H3N2) inherited five and six genes, respectively, from the cold-adapted parent, and hemagglutinin and neuraminidase genes from the wild-type strains.