  We investigate the possibility to measure the spin content carried by the
different quark flavors in a nucleon by means of polarized deep inelastic
scattering with $W^\pm$ exchange at HERA. Such measurements require a polarized
proton beam. The expected inclusive and semi inclusive asymmetries are sizable
and for realistic luminosities the expected statistical accuracies are good
enough to extract new and relevant information on the valence quark and the
strange sea distributions.
