HA-lipid spheres or "virosomes' were prepared using neutral or negatively charged, but not positively charged, phospholipids. Virosomes were similar in size and shape to native virus particles although the HA subunites were at least twofold less numerous on the virosomes. The HA subunites were attached by their narrow end to the lipid bilayer, and could be removed by digestion with bromelain. However, HA subunits released from intact virus by digestion with bromelain, which removed the hydrophobic tail of the molecule, could not attach to liposomes. Measurements of HA spikes before (mean length 14.2 +/- 0.9 nm) and after attachment to liposomes (mean length 13.3 +/-0.7 nm) and examination of freeze-fractured virosomes indicated that the HA did not penetrate deeply into the lipid bilayer. Similarly, HA subunits did not penetrate deeply into the lipid of virus particles. NP and M proteins could be attached to liposomes but could not be visualized by electron microscopy. Virosomes were taken up by Vero cells by viropexis with no evidence of fusion. Incorporation of HA or NP on to virosomes resulted in increased immunogenicity compared to free HA subunits or NP respectively. This adjuvant activity was not apparent in simple mixtures of HA liposomes. The antibody induced by HA subunits, virions and virosomes reacted similarly with strain-specific (SS) antigenic determinants of the haemagglutinin.