Abstract: The invention describes a vector including a nucleotide sequence encoding an HIV Gag polypeptide for use in the production of HIV-1 Gag virus-like particles. The vector may be a plant vector, for example, a tobacco mosaic virus vector such as the pBSG1057 vector or a tobacco etch virus vector. The vector may also be an Agrobacterium tumefaciens containing a T-derived plasmid construct. Alternatively, the vector may be an insect vector such as a baculovirus vector. HIV-1 Gag virus-like particles are also described, as is the use of the virus-like particles in a vaccine for use in the treatment or prophylaxis of HIV infection in a mammal, the vaccine including virus-like particles of proteins or polypeptides substantially as described above.