In a search for antituberculous compounds of low toxicity, the isonicotinylhydrazones of monohydroxybenzaldehydes and of the corresponding aldehydophenoxyacetic acids were prepared. When testedinνiνo, the isonicotinylhydrazones of 2-formyl-phenoxyacetic acid and of 6-methoxy-2-formyl-phenoxyacetic acid showed the most marked antituberculous activity combined with low toxicity. The preparation of 6-methoxy-2-formyl-phenoxyacetic acid and of the isonicotinylhydrazone of 2-formyl-phenoxyacetic acid are described as representative examples. The results of microanalysis include data which define the antituberculous isonicotinylhydrazones of 2-hydroxybenzaldehyde, 4-hydroxybenzaldehyde, and 2-hydroxy-3-methoxybenzaldehyde (o-vanillin).