Lipophilic methotrexate (MTX) and 3',5'-dichloromethotrexate (DCM) diesters were prepared by HCl-catalyzed esterification or by neutral esterification using cesium carbonate and an alkyl or aralkyl halide in Me2SO. The products were tested for in vivo antitumor activity against L1210 leukemia in mice to test whether all MTX and DCM diesters are therapeutically equivalent in this species. Contrary to what has been found with simple primary dialkyl esters, ortho-substituted dibenzyl esters of MTX produce longer survival on a q3dX3 schedule than does MTX itself and show a dose-sparing effect comparable to that of MTX at shorter treatment intervals. Thus, MTX bis(6-chloropiperonyl) ester at an MTX-equivalent dose of 5.5 mg/kg gave a +88% increase in median life span (ILS), whereas for MTX a +88% ILS required 30 mg/kg. When the MTX-equivalent dose of MTX bis(6-chloropiperonyl) ester was increased to 40 mg/kg, a +167% ILS was observed, as compared with a +100% ILS with 60 mg/kg of the parent drug. High activity (greater than 100% ILS) was likewise shown by the bis(2,5-dimethylbenzyl), bis(2,6-dichlorobenzyl), and di-3-picolyl esters of MTX and by the bis(1-methylbutyl) ester of DCM. The results of this study indicate that MTX and DCM esters are not therapeutically equivalent in mice, despite the high serum esterase activity in this species, and that an up to 10-fold reduction in total administered dose on the q3dX3 schedule is feasible by this approach.