  We calculate the single spin asymmetry in the inclusive pion production in
proton-proton collisions. We generate the asymmetry at the level of
fragmentation function (Collins effect) by the Lund coloured string mechanism.
We compare our results with the Fermilab E704 data from polarized proton-proton
collisions at 200 GeV. We show that the transversely polarized quark densities
at high Bjorken x strongly differ from these predicted by the SU(6) proton wave
function.
