Quantum violation of the equivalence principle in Brans - Dicke theory

We show that quantum fluctuations should violate the weak equivalence principle in the Brans - Dicke theory, which implies that the Jordan frame can no longer be interpreted as physical. In the Pauli frame the violation of the equivalence principle could be attributed to the existence of the dilatonic fifth force. We argue that the present fifth force experiments could put a much more stringent constraint on the Brans - Dicke coupling constant than accepted so far.