  We report recent studies of the Asymmetry Analysis Collaboration (AAC) on
polarized parton distribution functions (PDFs). Using the data on the spin
symmetry A_1 in deep inelastic lepton scattering, we investigate optimum
polarized PDFs. Their uncertainties are estimated by the Hessian method. The
uncertainties are large for the polarized antiquark and gluon distributions. We
discuss the role of accurate SLAC-E155 proton data on the determination of the
PDFs. The obtained distributions are compared with other parametrization
results.
