Multi-cycle replication and plaque formation of influenza A and B viruses and cleavage activation of their hemagglutinin (HA) by an endogenous protease(s) were examined in two MDCK cell lines, MDCK(-) and MDCK(+). No exogenous trypsin was required for multi-cycle replication and plaque formation of all the influenza A viruses tested in the MDCK(+) cell, while those of the viruses in the MDCK(-) cell were completely trypsin-dependent. In both cell lines, on the other hand, influenza B viruses grew well in the absence of trypsin. The capability of multiple replication and plaque formation of the influenza viruses correlated with cleavage of the HA precursor (HA0) to HA1 and HA2, indicating that both cell lines express an HA activating endoprotease(s); that of the MDCK(+) cell activates the HA of influenza A and B viruses, and that of the MDCK(-) cell does only the HA of influenza B virus. Furthermore, the protease of the MDCK(+) cell was strongly suggested to be present on the cell surface and a serine protease. The MDCK(+) cell would be useful for isolation of influenza viruses from clinical specimens and for screening of protease inhibitors for anti-influenza virus drugs.