  The fermion determinant in an instanton background for a quark field of
arbitrary mass is determined exactly using an efficient numerical method to
evaluate the determinant of a partial wave radial differential operator. The
bare sum over partial waves is divergent but can be renormalized in the minimal
subtraction scheme using the result of WKB analysis of the large partial wave
contribution. Previously, only a few leading terms in the extreme small and
large mass limits were known for the corresponding effective action. Our
approach works for any quark mass and interpolates smoothly between the
analytically known small and large mass expansions.
