Reproduction of cold-adapted (ca) strains of influenza virus in the lungs of white mice after separate and combined inoculation and the properties of isolates derived from the infected animals were studied. It was shown that after combined inoculation with ca and ts strains A/Leningrad/134/17/57 (H2N2) and A/PR/8/59/1 (H1N1) ca recombinants could develop loosing some ts mutations and possessing (unlike the master strains) pneumo-virulence for mice. All the pneumo-virulent reassortants inherited hemagglutinin from the ca A/PR/8/59/1 strain and PB1 protein from the ca A/Leningrad/134/17/57 strain. The results indicate that it is unsafe to construct live recombinant divaccines by combining the recombinants produced from different donors of attenuation.