  We prove a long exact sequence in KK-theory for both full and reduced
amalgamated free products in the presence of conditional expectations. In the
course of the proof, we established the KK-equivalence between the full
amalgamated free product of two unital C*-algebras and a newly defined reduced
amalgamated free product that is valid even for non GNS-faithful conditional
expectations. Our results unify, simplify and generalize all the previous
results obtained before by Cuntz, Germain and Thomsen.
