The synthesis and chemical behaviour of pyrimidothiazine derivatives with a carboxyl group in position 7 [compounds (II) and (III)] or in position 6 [compounds (IV) and (V)] are described. It has been shown that these products, analogous with the isosteric benzothiazines, have the 5H-pyrimidothiazine structure. Compounds (II), (III) and (IV) show immunological activity, inhibiting passive cutaneous anaphyllaxis in the rat.