  Two Azumaya algebras with involutions are considered over a regular local
ring. It is proved that if they are isomorphic over the quotient field, then
they are isomorphic too. In particular, if two quadratic spaces over such a
ring are similar over its quotient field, then these two spaces are similar
already over the ring. The result is a consequence of a purity theorem for
similarity factors proved in this text and the known fact that rationally
isomorphic hermitian spases are locally isomorphic.
