Liposomes were loaded with fluorescence-labelled serum albumin and their interaction with mouse spermatozoa was examined in a fluorescence microscope. In the presence of polyethylene glycol 1500, fluorescence became unevenly distributed along the entire body of mature spermatozoa, mostly in the form of aggregates, indicating adsorption of liposomes to the surface of the cells. The fluorescence could be largely removed by washing the cells. In the case of immature sperm cells, fluorescence was evenly distributed on the cells and was not removed by extensive washing, suggesting an internalization of the labelled albumin. A more direct evidence for the entrance of liposome-entrapped macromolecules into the immature sperm cells was provided by autoradiography: radioactivity was found in discrete spots in spermatocytes upon their incubation with liposomes containing isotopically labelled DNA.