  Recent WMAP observations indicate that the primordial abundance of lithium
should have been 2-4 times higher than the value measured in low-metallicity
stars. We show that this discrepancy can be explained by the process of
ambipolar diffusion in protostellar clouds, which leads to depletion of
elements with low ionization potential, such as lithium and sodium. In
high-metallicity stars, [Fe/H]>~-1, the depletion of lithium due to this
process is found to be negligible, but for [Fe/H]<~-1.5 the lithium abundance
falls by a factor of ~3, consistent with WMAP observations.
