0966-842X/99/$-see front matter© 1999 Elsevier Science. All rights reserved. PII: S0966-842X (99) 01453-5 peptide. This finding might shed light on some previously unexplained observations. The fusion peptide is the most conserved part of HA among all influenza viruses, and it has long been known that minor deviations from its correct structure, for example the absence of the amino-terminal glycine and substitution of other glycine residues by glutamic acid, prevent fusion or affect pH dependance14, 15. It therefore appears that exact fitting of the amino-terminal part of HA2 into the cavity is an essential step in the sequence of events leading to fusion. This concept is also supported by the observation that the infectivity of paramyxoviruses can be inhibited by peptides mimicking the fusion domain of these viruses16. It will be interesting to see if such peptide analogues will also interfere with influenza virus replication. If so, they might become useful for the treatment of influenza, as has already been suggested for protease inhibitors1.