ALVAC recombinants have been administered to humans and animals by parenteral and oral routes without giving signs of replication, systemic dissemination or severe reaction. In principle, it should be impossible for canarypox recombinants to disseminate in the environment as they would not be synthesised in mammalian cells as complete virus. Canarypox vectors have been safe for humans, in whom there has been no evidence of replication, but more work needs to be done to prove absence of replication. Recombinants are immunogenic by the intramuscular and subcutaneous routes. They are also immunogenic when given orally, but the dose required is still under study. Canarypox recombinants effectively prime the immune system for induction of antibodies and CD8 cell-mediated cytotoxicity by protein antigens. Antibody responses are not influenced by prior inoculation of canarypox, of subunit vaccine corresponding to the gene insert, or of vaccinia. Canarypox virus is attenuated for canaries, in which species it is already widely used. In principle, it is non-infectious for humans or other mammals. It may be infectious for other birds.