Various substituted 2-(5-substituted-2-oxoindolin-3-ylidene)-N-substituted hydrazine carbothioamide 4a–g and 2-(5-substituted-1-(4-substituted benzyl)-2-oxoindolin-3-ylidene)-N-substituted hydrazine carbothioamide 5a–k were synthesized. The compounds were evaluated for their cytostatic activity against human Molt4/C8 and CEM T-lymphocytes as well as murine L1210 leukemia cells. Several of these compounds were endowed with low micromolar 50%-inhibitory concentration (IC50) values, and some were virtually equally potent as melphalan. The most potent inhibitors against the murine leukemia cells were also most inhibitory against human T-lymphocyte tumor cells. 2-(5-fluoro-1-(4-fluorobenzyl)-2-oxoindolin-3 ylidene)-N-p-tolylhydrazine carbothioamide (5b) emerged as the most potent cytostatic compound among the tested compounds. The encouraging cytostatic data provide an adequate rationale for further modification of these molecular scaffolds.