BCG, an attenuated vaccine strain of Mycobacterium bovis, has been engineered for use as an antigen delivery vehicle. Various genetic and molecular biological techniques were recently developed for use in the manipulation of mycobacteria. Vectors for the expression of foreign antigens in BCG have been constructed and used to express genes from bacteria, protozoa and viruses. Here, an overview of recombinant BCG is given, with emphasis on experiments with three different foreign antigens. Antibody, T-helper and cytotoxic T cell responses can be induced in mice following immunization with recombinant BCG (rBCG). In addition, vectors allowing expression of foreign antigens on the surface of rBCG have been constructed; use of these systems can lead to enhanced antibody response against some antigens. Finally, expression in BCG of a protein from the parasite Leishmania can provide protection against a virulent Leishmania challenge in mice.