  We show that the spin asymmetry in semi-inclusive deep inelastic scattering
between a longitudinally polarized lepton beam and a transversely polarized
proton target is sensitive to the proton transversity function. The asymmetry,
which is twist 3, is estimated to be more than 10% under the most favourable
conditions. The experiment we suggest is feasible at facilities like DESY, CERN
and JLAB.
