Several 2,4-dinitrophenylseleno-carboxylic acids, a number of unsubstituted and 4-substituted benzylselenocarboxylic acids, and some phenyletbylseleno-carboxylic acids have been investigated with regard to their potency in the prevention of dietary liver necrosis. The potency was strongly influenced by the structure of the alkanoic moiety of the molecule and the distance between the carboxyl group and the selenium atom. An alternating effect of the length of the carboxylic acid carbon chain was obvious in 2,4-dinitrophenylseleno-carboxylic acids, unsubstituted benzylseleno-n-carboxylie acids, and 4-nitrobenzylseleno-carboxylic acids, as previously described for aliphatic monoseleno-dicarboxylic acids and selena-carbexylie acids. Presence of a quaternary carbon atom in the chain eliminated potency almost entirely. Substituents in the aromatic ring also exerted a strong influence. In the benzylseleno-carboxylic acid series, introduction of methyl or nitro groups in the 4 position of the ring decreased biopotency. The effect of bromine substitution in this position was basically different: The alternating effect was abolished, and a pattern of activity arose which was similar to that of the diseleno-dicarboxylic acids. The results are discussed in the light of structural activity relationships found previously in aliphatic mono-and diseleno-diearboxylic acids and selena-carboxylic acids.