The polarization tensor is calculated which originates from interaction of
a photon with the electron-positron field. The effects of multiple scatteri
ng of electrons and positrons in a medium side by side with external fields
are included. The corresponding general representation of polarization ten
sor is found in the form of two-dimensional integral. The both effects may
be essential for propagation of high-energy photons in oriented single crys
tals. (C) 2001 Published by Elsevier Science B.V.