The ectodomain of HA is translocated into the ER lumen, where it undergoes a series of folding events mediated by the formation of disulfide bonds and glycosylation reactions. The formation of a discrete intermediate species of highly folded monomeric protein preceeds trimerisation. The folding process is efficient and rapid, with greater than 90% of the protein trafficked to the golgi apparatus; and mature HA0 subunits appearing in a matter of a few minutes. Calnexin and calreticulin have been identified as cellular lectins which interact transiently with newly synthesized HA by attaching to partially trimmed N-linked oligosaccharides (Herbert et al., 1997), facilitating correct folding of the HA molecule.