  We present intermediate dispersion (0.7-2.2 \AA ~pix$^{-1}$) optical
spectroscopic observations aimed at applying the ``Lithium Test'' to a sample
of ten brown dwarf candidates located in the general field, two in young open
clusters, and two in close binaries. We find evidence for strong Li depletion
in all of them, and thus infer lower mass limits of 0.065~M$_\odot$, depending
only slightly ($\pm$0.005~M$_\odot$) on the interior models. None of the field
brown dwarf candidates in our sample appears to be a very young (age
$<$~10$^8$~yr) substellar object. For one of the faintest proper motion
Pleiades members known (V=20.7) the Li test implies a mass greater than
$\sim$0.08~M$_\odot$, and therefore it is not a brown dwarf. From our spectra
we estimate spectral types for some objects and present measurements of Halpha
emission strengths and radial velocities. Finally, we compare the positions in
the H-R diagram of our sample of brown dwarf candidates with the theoretical
region where Li is expected to be preserved (Substellar Lithium Region). We
find that certain combinations of temperature calibrations and evolutionary
tracks are consistent with the constraints imposed by the observed Li depletion
in brown dwarf candidates, while others are not.
