We discuss effects related to the fact that the final state particles of a
reaction e+ e- -> t \bar{t} H are actually produced and they decay off mass
shell. For the intermediate mass Higgs boson, which decays preferably into a
b\bar{b}-quark pair, the reaction will be observed through reactions with 8
fermions in the final state. Such reactions, already in the lowest order of the
standard model, receive contributions typically from a few dozen thousands of
the Feynman diagrams, the vast majority of which constitute background to the
signal of associated production of the top quark pair and Higgs boson. In order
to illustrate pure off mass shell effects we neglect the background
contributions and compare the `signal' cross section with the cross section in
the narrow width approximation for e+ e- -> b u \bar{d} \bar{b} mu- \bar{nu}_mu
b \bar{b}, which is one of possible detection channels of the associated
production of the top quark pair and Higgs boson at a linear collider.Comment: 11 pages, 4 figures, version published in Acta Physica Polonica
