  The strength of the Yukawa coupling of the leptons and quarks with the Higgs
boson is uniquely determined by the Higgs mechanism for generating masses in
the Standard Model. The top-Higgs coupling can be measured directly in the
process e^+e^- --> t tbar H. This process therefore provides a fundamental test
of the Higgs mechanism. Extending earlier analyses carried out in the Born
approximation, we have determined the cross section for this process including
QCD corrections, which turn out to be important.
