Virosomes are reconstituted viral envelopes which lack the genetic material but retain the cell entry and membrane fusion characteristics of the virus they are derived from. Thus, influenza virosomes are taken up by cells via receptor-mediated endocytosis, which directs the particles to the endosomal cell compartment. Subsequently, the virosomal membrane fuses with the endosomal membrane induced by the mildly acidic pH within the endosomes. This fusion process establishes continuity between the lumen of the virosome and the cell cytosol. Upon interaction of virosomes with antigen-presenting cells (APCs), protein antigens encapsulated within virosomes will be delivered to the cell cytosol, and thus, into the MHC class I presentation pathway. Indeed, virosome-mediated delivery of antigens in vivo results in efficient priming of a class I MHC-restricted cytotoxic T lymphocyte (CTL) response.