  We discuss Lambda polarization in semi-inclusive proton-proton collisions,
with one of the protons longitudinally polarized. The hyperfine interaction
responsible for the $\Delta$-$N$ and $\Sigma$-$\Lambda$ mass splittings gives
rise to flavor asymmetric fragmentation functions and to sizable polarized
non-strange fragmentation functions. We predict large positive Lambda
polarization in polarized proton-proton collisions at large rapidities of the
produced Lambda, while other models, based on SU(3) flavor symmetric
fragmentation functions, predict zero or negative Lambda polarization. The
effect of $\Sigma^0$ and $\Sigma^*$ decays is also discussed. Forthcoming
experiments at RHIC will be able to differentiate between these predictions.
