New 2-benzylamino-3,4-dihydroxypyrrolidines bearing aromatic and aliphatic amido side chains have been prepared. The influence of the amido substituents on the inhibitory activity of these diamines toward 24 commercially available glycosidases was determined. The most potent and selective alpha-mannosidase inhibitor (6d) (N-[(2R)-2-({[(2R,3R,4S)-3,4-dihydroxypyrrolidin -2-yl]methyl} amino)-2-phenylethyl]-3-bromobenzamide) of these series was also the most potent inhibitor of the growth of human glioblastoma cells.