From the reassortment experiments between A/Aichi/4/92 and A/WSN/33 (WSN) (H1N1) viruses, two different phenotype viruses which contained the haemagglutinin (HA) gene from A/Aichi/4/92 virus and the neuraminidase (NA) gene from WSN virus were obtained. PW13 and PW15 viruses agglutinated chicken red blood cells (CRBC), while PW10 and PW70 viruses did not. However, the expressed HA proteins of these viruses did not adsorb CRBC. The difference in gene constellation between PW13, PW15 and PW10, PW70 viruses was the membrane protein (M) gene. The former two had the M gene from A/Aichi/4/92 virus and the latter two had that from WSN virus. In PW15-infected cells, haemadsorption of CRBC was observed 30 min later than that of goose red blood cells and the M1 protein migrated from the nucleus to the cytoplasm 30 min earlier than adsorption of CRBC was observed. On the other hand, in PW10-infected cells, haemadsorption of CRBC was not observed through the virus replication and the M1 protein stayed in the nucleus after HA and NA activities reached maximum levels. Co-expression of the M and the HA proteins of A/Aichi/4/92 virus did not help the HA protein gain the ability to adsorb CRBC. However, neuraminidase treatment of COS cells expressing the HA protein of A/Aichi/4/92 virus or MDCK cells infected by PW10 virus restored the ability to adsorb CRBC. We discussed the possibility that the M1 protein helped the NA protein in its role to modify the HA protein on the cell surface.