Polarization of Valence and Sea Quarks in the Proton

Analysis was performed of semi-inclusive and inclusive spin asymmetries determined from the polarized deep inelastic scattering by the Spin Muon Collaboration. Combined analysis of data for polarized deuterium and hydrogen targets allows for separate determination of spin carried by valence {\it u} and {\it d} quarks and non-strange sea quarks as a function of $x_{Bj}$ in the range $0.006<x_{Bj}<0.6$. It was found that polarization of valence {\it u} quarks is positive and of valence {\it d} quarks is negative, whereas the sea polarization is small and consistent with zero within errors.