We present intermediate dispersion (0.7-2.2 A/pixel) optical spectroscopic observations aimed at applying the 'lithium test' to a sample of 10 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 solar mass, depending only slightly (+/- 0.005 solar mass) on the interior models. None of the field brown dwarf candidates in our sample appears to be a very young (age less than 108 yr) substellar object. For one of the faintest proper motion Pleiades members known (V = 20.7) the Li test implies a mass greater than approximately 0.08 solar mass, and therefore it is not a brown dwarf. From our spectra we estimate spectral types for some objects and present measurements of Hα emission strengths and radial velocities. Finally, we compare the positions in the Hertzsprung-Russell (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.