A reassortant virus possessing RNA segment 7, which codes for the M1 and M2 proteins, of the avian influenza A/Mallard/New York/6750/78 (H2N2) virus and the other seven RNA segments of the human influenza A/Udorn/307/72 (H3N2) virus had been shown previously to be markedly restricted in replication in the respiratory tract of squirrel monkeys. In contrast, a reassortant possessing segment 7 of another avian influenza virus, A/Pintail/Alberta/119/79 (H4N6), and the seven other RNA segments from the A/Udorn/72 virus was not restricted. The nucleotide and deduced amino acid sequence of the RNA segment 7 of each virus was determined to identify the structural basis for the attenuation phenotype specified by RNA segment 7 of the A/Mallard/78 virus. Analysis of the deduced amino acid sequences revealed only two amino acid differences in the M1 protein and one difference in the M2 protein, suggesting that the attenuation phenotype of a reassortant virus possessing segment 7 of the A/Mallard/78 virus may be specified by one to three amino acids. Reassortant viruses possessing RNA segment 6, which codes for the nucleoprotein, of either avian influenza virus and the other seven RNA segments of a human influenza virus were also restricted in replication in squirrel monkeys. A comparison of the deduced amino acid sequences of the two avian nucleopeoteins demonstrated only three amino acid differences indicating that these two avian viruses possess NP genes that are highly related. The high degree of relatedness of both the NP and M proteins of these two avian viruses contrasts with their divergent surface antigens.(ABSTRACT TRUNCATED AT 250 WORDS)