Mice can be protected against several species of lethal malaria infection by vaccination¹, and their recovery correlates well with increased anti-malarial antibody levels, particularly IgG (ref. 2). However, there is also a good correlation between protection by vaccines and priming for delayed-type hypersensitivity in the skin^3–5, although there is no obvious explanation for this effect. We now report an apparent relationship between protection and a cell-mediated immune response involving the migration of various types of cell capable of killing malaria parasites in vitro to the liver. We suggest that the effect of vaccination is to bring together parasites, specific antibody and nonspecific cytotoxic cells, and that the liver may be a major site for their interaction.