We postulate (Table I) that ReA is an antigen or immune complex induced condition caused by chronic intracellular bacterial infection at a distant site. The main predisposing factor is a failure to resist this infection. If the bacteria happen to carry MF, the inflammation is exacerbated in B27 positive patients. In contrast AS occurs in individuals who lack immunity to MF and eventually become infected by an intracellular organism which synthesizes it (virus or plasmid?). HLA-B27 acts only at the site of inflammation.