We discuss the contribution of pure gravitational waves to the cosmic microwave background radiation (CMBR) anisotropy and polarization. It is found that the large-scale polarization of the CMBR is less than 1% of the anisotropy for an universe with standard recombination. The effect of early matter reionization will enhance the CMBR polarization to a 10% level. We have also computed the CMBR polarization for an universe having high baryon density and gravitational waves with a tilted spectrum and found that further enhancement of the polarization is possible. We conclude that measuring the polarization of the CMBR on large angular scales can probe the ionization history of the early universe and set constraints on cosmological models.