An immunogenic reagent which produces an immune response which is protective against Bacillus anthracis, said reagent comprising one or more polypeptides which together represent up to three domains of the full length Protective Antigen (PA) of B. anthracis or variants of these, and at least one of said domains comprises domain 1 or domain 4 of PA or a variant thereof. The polypeptides of the immunogenic reagent as well as full length PA are produced by expression from E . coli. High yields of polypeptide are obtained using this method. Cells, vectors and nucleic acids used in the method are also described and claimed.
