  Frenkel and Reshetikhin introduced screening operators related to
q-characters of finite dimensional representations of quantum affine algebras.
We propose t-analogs of screening operators related to Nakajima's
q,t-characters with the same properties of symmetry. Nakajima introduced
non-commutative rings, so we propose bimodules. Our construction uses only
combinatorial definition of q,t-characters, and therefore can be extended to
the non-simply laced case.
