The matrix protein (M1) of influenza virus plays a central role in viral replication. In relation to viral growth and morphology, we studied the RNP-binding activity of M1s from high-growth strain A/Puerto Rico/8/34 (A/PR8/34) and the relatively low-growth wild-type strain A/Nanchang/933/95. The RNP-binding strength of M1 was studied by disruption of M1 from M1/RNP complexes with salt and acidic condition. Our results indicated that binding of M1 of high-growth A/PR8/34 was more difficult to break than the binding of M1 of low-growth A/Nanchang/933/95. Consistent with the presence of M1 in A/PR8/34, binding of M1 of Resvir-9, a reassortant containing P, M, and NS genes from A/PR8/34 and the rest of genes from A/Nanchang/933/95 and retaining relative high-growth characteristic, was relatively difficult to break than the binding of M1 of A/Nanchang/933/95. Physical properties of morphological features of these viruses were studied by velocity sucrose gradient centrifugation and transmission electron microscopy of purified viral particles, and by immunofluorescence staining of hemagglutinin expressed on the surface of infected cells. The results demonstrated that high-growth strains, A/PR8/34, and a relative high-growth reassortant, Resvir-9, had characteristics associated predominantly with spherical particles, while the low-growth strain, A/Nanchang/933/95, had characteristics of filamentous particles. These studies indicate that the binding between M1 and RNP complex might determine viral growth and morphology.