A great deal of effort is directed towards developing a sporozoite vaccine. In the last decade, target antigens have been identified and cloned, and a number of vaccine trials undertaken in both humans and laboratory animals. One of the problems facing us is the lack of widespread immunogenicity of vaccine candidates. This relates, at least in part, to genetic factors in the vaccinee. Genes within the major histocompatibility complex (MHC) which restrict some sporozoite-specific responses have been identified. Antigenic diversity also contributes to the difficulty of developing a successful vaccine. This brief report provides some background information for these topics.