Phenotypical properties of single-gene reassortants of attenuated cold-adapted strain A/Leningrad/135/47/57 (H2N2) and strain A/PR8/34 virulent for laboratory animals were studied. Only the group of reassortants inheriting NS gene from cold-adapted virus was fully attenuated for various animals species, similarly as reassortants with 6/2 genomic formula containing all the 6 internal protein genes from strain A/Leningrad/134/47/57. Reassortant 25A-1 single-gene for NS was temperature-sensitive (ts) on mammalian cells but formed plaques at 40 degrees C on chicken kidney cells. Reassortants with genomic formula 6/2 were temperature-sensitive in all types of cells used. Reassortant 25A-1 could synthesize normal amounts of polypeptides in MDCK cells at 39 degrees C, whereas protein synthesis of reassortants with 6/2 genomic formula was noticeably reduced at this temperature. Hence, a similar level of attenuation of both reassortant groups appears to be due to various molecular mechanisms. Possible role of NS2 gene mutation in attenuation of strain A/Leningrad/134/47/57 and its reassortants is discussed.