The prospects for vaccine development have been transformed by the advent of hybridoma technology for the production of specific monoclonal antibodies and recombinant DNA technology which permits the in vitro expression of genes coding for protective antigens of pathogenic organisms. These techniques are leading to the refinement of existing vaccines and the production of new vaccines against organisms which either cannot be cultured in vitro at all, e.g. hepatitis-B, or can be cultured only on a limited scale, e.g. plasmodial species. The techniques allow for the novel production of both live and dead vaccines. New live vaccines are produced using either site-directed mutagenesis or insertion of genes into hosts such as vaccinia virus, which is a potential carrier of polyvalent vaccines. New subunit vaccines are produced by expression of appropriate genes in E. coli, yeast cells or mammalian cell lines and some, e.g. hepatitis-B surface antigen (HBsAg) are at an advanced stage of development. Recombinant subunit vaccines are also being produced against the sporozoite and blood stages of malaria. Problems, in regard to the development of malaria vaccines, concern the antigenic plasticity of plasmodia and the need to induce both antibody and cell-mediated immunity; nevertheless, the work shows considerable promise of success.