S-(p-Bromobenzyl)glutathione is a competitive inhibitor of human glyoxalase I which is part of the cytosolic glyoxalase system. It may be delivered into the cytosol of cells by diesterification wherein it is deesterified by cytosolic nonspecific esterases. S-(p-Bromobenzyl)glutathione diesters had antitumor activity in vitro and in vivo. The inhibition of human leukemia 60 cell growth in vitro by a series of alkyl and cycloalkyl diesters of S-(p-bromobenzyl)glutathione was investigated. For n-alkyl diesters, the n-propyl diester was the most potent derivative with a median growth inhibitory concentration GC₅₀ value of 7.77 ± 0.01 μM (N = 18). The most potent derivative was S-(p-bromobenzyl)glutathione cyclopentyl diester which had a GC₅₀ value of 4.23 ± 0.01 μM (N = 21) and also had antitumor activity in vivo.