  We show that there is a well-defined cap-product structure on the
Fintushel-Stern spectral sequence. Hence we obtain the induced cap-product
structure on the ${\BZ}_8$-graded instanton Floer homology. The cap-product
structure provides an essentially new property of the instanton Floer homology,
from a topological point of view, which multiplies a finite dimensional
cohomology class by an infinite dimensional homology class (Floer cycles) to
get another infinite dimensional homology class.
