It has previously been shown that influenza virus neuraminidase (NA) of the N9 subtype is unusual in that it possesses hemagglutinin activity as well as NA activity. Loss of red cell binding in certain escape mutants suggested that the hemagglutinating site is separate from the NA active site and involves at least two of the polypeptide loops found on the surface of the molecule (Webster et al., 1987. J. Virol. 61, 2910-2916). We have used site-directed mutagenesis to transfer the amino acids in these loops at positions 368-370 and 399-403 of N9 NA (A/tern/Australia/G70c/75), separately and together, into subtype N2 NA (A/Tokyo/3/67). The three mutant proteins were expressed from an SV40 transient expression system (Fuerst et al., 1986. Proc. Natl. Acad. Sci. USA. 83, 8122-8126). The mutant which contained both loops of N9 NA had acquired the hemagglutinin activity of N9. The agglutinated red cells are released by the enzyme activity of N9 NA, indicating that the agglutination involves binding to sialic acid in the same configuration as does the parental N9 NA, and an inhibitor of NA did not affect hemagglutination, indicating that this site is separate from the NA site as in parental N9.