The potential of reconstituted Sendai viral envelopes containing only the fusion protein (F-virosomes) was evaluated for a targeted cytosolic delivery of lysozyme to human hepatoblastoma cells (HepG2) in culture. 125I-Lysozyme loaded into F-virosomes was used to monitor its fusion-mediated transfer to the HepG2 cells. Using fusion assay based on the transfer of water soluble probe, we have demonstrated the existence of aqueous connection between F-virosomes and target cells. Target specificity of the F-virosomes was ensured by the strong interaction between terminal β-galactose moiety of F protein and the asialoglycoprotein receptor on the membrane of HepG2 cells. Incubation of the loaded F-virosomes with cells resulted in fusion-mediated injection, as inferred from the ability of cells to internalize lysozyme in the presence of azide (an inhibitor of the endocytotic process). Binding as well as fusion of the F-virosomes to HepG2 cells was solely mediated by the F protein. Introduction of 125I-lysozyme into the HepG2 cells was confirmed by selective accumulation of acid and anti-body-precipitable radioactivity in the cytosolic compartment. The structural integrity of the internalized lysozyme was also assessed. The potential usefulness of F-virosomes with defined specificities as biological carrier for both in vitro and in vivo cytosolic delivery of macromolecules and drugs has been established.