  In this paper we study modular tensor categories (braided rigid balanced
tensor categories with additional finiteness and non-degeneracy conditions), in
particular, representations of quantum groups at roots of unity. We show that
the action of modular group on certain spaces of morphisms in MTC is unitary
with respect to the natural inner product on these spaces. In a special case of
category based on representations of the quantum group U_q sl_n at roots of
unity we show that in some of these spaces of morphisms (for U_q sl_2, in all
of them) the action of modular group can be written in terms of values of
Macdonald's polynomials of type A at roots of unity. This gives identities for
these special values, both known before (symmetry identity) and new ones.
  The paper contains a detailed exposition of the theory of modular categories
as well as construction of modular categories from representation of quantum
groups at roots of unity
