We have expressed the superantigen staphylococcal enterotoxin A (SEA) on the surface of bacteriophage as a fusion with the gene VIII protein (gVIIIp). This phage-displayed superantigen retains the properties inherent in the natural protein. It binds to MHC class II and activates T-cells bearing appropriate V beta regions. A flexible 5-amino acid linker sequence between the SEA molecule and the phage coat protein improved the production of functional phage-displayed SEA. Binding to MHC class II-expressing cells effectively selected SEA-phage from non-SEA-phage background. This indicates that this will be an effective method for selecting new specificities of superantigen from libraries of SEA mutants and for cloning of novel superantigens.