  Let X be a smooth projective variety of dimension n. If $p+q=n+1$ then Bloch
has defined a ${\bf G}_m$-biextension E over the product of the Chow groups
$CH^p_0(X)$ and $CH^q_0(X)$ of homologically trivial cycles. We prove that E is
the pullback of the Poincare biextension over the product of intermediate
Jacobians in characteristic zero. This is used to study various equivalence
relations for algebraic cycles. In particular we reprove Murres result that
Griffiths conjecture holds for codimension two cycles, i.e. every codim. two
cycle algebraically and incidence equivalent to zero has torsion Abel-Jacobi
invariant.
