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The influenza virus copies its genomic RNA in the nuclei of host cells, but the viral particles are formed at the plasma membrane. Thus the export of a new genome from the nucleus into the cytoplasm is essential for viral production. Several viral proteins, such as nucleoprotein (NP), RNA polymerases, and matrix protein 1 (M1), synthesized in the cytoplasm are imported into the nucleus and form a viral ribonucleoprotein complex (vRNP) with new genomic RNA. vRNP is then exported into the cytoplasm from the nucleus. It was found unexpectedly that the production of influenza virus was suppressed in Madin-Darby canine kidney cells at 41 degrees C, although viral proteins were synthesized, because nuclear export of vRNP is blocked by the dissociation of M1 from vRNP. It was also suggested that a certain protein(s) synthesized only at 41 degrees C inhibited the association of M1 with vRNP. The potential of heat-shock protein 70 (HSP70) as a candidate obstructive protein was investigated. Induction of HSP70 by prostaglandin A1 (PGA1) at 37 degrees C caused the suppression of virus production. The nuclear export of viral proteins was inhibited by PGA1, and M1 was not associated with vRNP, indicating that HSP70 prevents M1 from binding to vRNP. An immunoprecipitation assay showed that HSP70 was bound to vRNP, suggesting that the interaction of HSP70 with vRNP is the reason for the dissociation of M1.