  We use cosmological simulations of reionization to predict the effect of
large HII regions around individual high-redshift quasars on the possible
signal from the redshifted 21cm line of neutral hydrogen in the
pre-reionization era. We show that these HII regions appear as ``spectral
dips'' in frequency space with equivalent widths in excess of 3 mK MHz or
depths in excess of about 1.5 mK, and that they are by far the most prominent
cosmological signals in the redshifted HI distribution.
  These spectral dips are expected to be present in almost every line of sight.
If the spectral dips of a large enough sample of HII regions are well resolved
in frequency space, the distribution of line depth and equivalent width in
frequency with a known observing beamsize can be used to infer the HII region
size distribution and the mean difference in neutral hydrogen density between
the \HII regions (which may contain self-shielded neutral gas clumps) and the
surrounding medium, providing a powerful test for models of reionization.
