Previously, we reported that influenza virus-induced cell-mediated cytotoxicity (CMC) was largely due to its glycoproteins, hemagglutinin and neuraminidase (NA). These observations were based on the use of a single influenza virus strain, the A/Port Chalmers/3/73 (H3N2), and these were considered insufficient to generalize that all human influenza virus NAs augment CMC. Therefore, antigenically different NAs of human influenza strains were used to study whether (a) all NAs possess the potential to stimulate NK activity and (b) does the enzymatic activity of NA play a role in the CMC stimulation. Biologically active preparations of N1 subtype NA (A/USSR/90/77 (H1N1) and N2 subtype NAs (A/Aichi/2/68 (H3N2) and A/Port Chalmers) were evaluated for NK activity stimulation in an overnight radiolabeled chromium-release assay consisting of human peripheral blood lymphocytes and K562 target cells. The level of CMC stimulation was the same at equivalent protein concentrations with all the NAs tested. The addition of homologous NA-monospecific antibody almost completely reduced the CMC stimulation, while the addition of homosubtypic antibody reduced the CMC by 56-75%. However, in the presence of heterosubtypic monospecific antibody, NA-augmented CMC was reduced by 27-47% in most experiments. The results suggest that the CMC stimulation site is probably the same in all NAs tested. This putative site is thermo-resistant and is independent of the conformational change of the NA molecule. Furthermore, it is distinct from the enzymatic and probably from the antigenic sites.