  We study the Sivers function for valence u and d quarks in the proton within
the instanton model for QCD vacuum, adopting the MIT bag model wave functions
for quarks. Within approaches based on perturbative one-gluon final state
interactions a non-zero value of the Sivers function is related to the presence
of both S and P wave components in quark wave functions. We show that the
instanton-induced chromomagnetic, nonperturbative interaction leads to very
specific spin-spin correlations between the struck and spectator quarks,
resulting in a non-trivial flavour dependence of the Sivers function.
Comparison of the obtained Sivers functions with phenomenological
parameterizations is discussed.
