We therefore focused our study on the HA and NA functions of strains with differing morphological phenotypes. We took advantage of two M1 point mutants selected during serial adaptation of rPR8 virus to an animal host. By measuring particles in electron micrographs, the R101G mutant was previously shown to comprise 41% filamentous particles, while the N87S had 16% filaments and the rPR8wt virus had 4% filaments. Our approach, focused on the surface of the virion, assumes that the internal components of spherical and filamentous particles are similar. Our results indicating comparable infectivity and RNA content per HAU for spherical and filament-producing strains supports this DAPT assumption. Similar results were also reported by Roberts et al. for the A/Udorn/301/1972 strain. Nevertheless, the literature contains conflicting reports on the genomic content of filaments versus spheres. Early studies suggested that filamentous virions could be polyploid or contain more RNA than their spherical counterparts. In contrast, a recent cryoelectron tomography study has shown that many longer filaments produced by A/Udorn/301/1972 virus lack RNPs. Lastly, sectioning TEM and cryo-electron tomography studies have shown that filamentous virions contain a single copy of the viral genome located at the apical tip of the budding virion. These apparently contradictory results can be partially reconciled by noting that the absence of genomes from filamentous particles appears to apply mainly to very long filaments. In some cases, IAV strain specific differences in the properties of filaments may also play a role. Our observations through two independent functional assays show that the two filament-producing rPR8 mutants have higher NA activities than the spherical rPR8wt virus. Replacement of the PR8 M segment with that of the filamentous 2009 pandemic strain A/Netherlands/602/2009 was also shown to increase both filament production and NA activity compared to the rPR8wt virus. Now we show that significant increases in NA activity can be conferred through a single point mutation that changes virion morphology, thereby strengthening the causal link between morphology and NA activity. We predict that the increased NA activities associated with filament-containing virus preparations are due to greater numbers of NA proteins adorning the surface of filaments compared to spheres. We were not able to test this prediction robustly, however, due to limitations in the sensitivity of our Western blot assay. An alternative mechanism by which morphology could impact NA activity relates to the distribution of NA molecules on the virion surface. If filaments and spheres differ in terms of the positioning of NA on the particle, increased neuraminidase activity could be due to a cooperative effect mediated by greater NA protein clustering on filamentous virions. Consistent with this idea, clustering of NA at the tip of the virus particle proximal to the cell membrane has been reported.