The effects of various structural modifications on the antineoplastic activity of the benzenesulfonylhydrazone of 2-formylpyridine N-oxide have been ascertained in mice bearing either Sarcoma 180 or leukemia L1210. To accomplish this a variety of derivatives substituted at the aldehyde proton, the aryl ring, and the 4 position of the pyridine nucleus were synthesized. Antineoplastic activity was retained when nitro, amino, chloro, bromo, fluoro, and methoxy groups were introduced into either the meta or para positions of the phenyl ring of the parent compound. In addition, substitution of the terminal phenyl group by a pyridine ring or by a bulky aromatic ring such as alpha-naphthyl, beta-naphthyl, or fluorenyl did not abolish the marked antitumor activity expressed by this class of agents. Insertion of a nitro function or a morpholino group in the 4 position of the pyridine nucleus of the benzenesulfonylhydrazone of 2-formylpyridine N-oxide resulted in two potent anticancer agents, while the introduction of a chloro function in the 4 position led to a pronounced decrease in biological activity. Furthermore, the essentiality of the aldehydic proton for tumor-inhibitor activity was demonstrated by the inactivity of two derivatives in which the aldehydic proton was replaced by a methyl group or by an oxygen atom.