Vaccines need to activate antigen presenting cells, overcome genetic restriction in T-cell responses and elicit both T and B memory cells. In order to produce recombinant vaccines which can do this, considerable effort has been put into developing particulate antigen presentation systems to generate polyvalent, high molecular weight antigens which should maximally stimulate the immune system. One such antigen-carrier system is based on the ability of a protein encoded by the yeast retrotransposon, Ty, to self-assemble into virus-like particles (VLPs). Ty-fusion proteins retain this ability to form particles and a range of hybrid VLPs carrying a variety of heterologous antigens have been produced and shown to elicit potent immune responses. Hybrid VLPs carrying human immunodeficiency virus (HIV) antigens stimulate the three main components of the immune system, namely antibody synthesis, T-cell proliferative responses and cytotoxic T-lymphocyte (CTL) responses.