AT the time of the discovery of the selective herbicidal activity of (i) the substituted phenoxy-acetic acids1,2and (ii) the arylcarbamic esters3,4, many other chemical types were examined, and 2-benzoylbenzoic acid and its derivatives received considerable attention because of their marked biological activity5. The investigation of this type was at the time abandoned because of the superior promise of the phenoxyacetic acid derivatives as agents for the practical control of crop weeds. It is felt desirable, nevertheless, to place our observations on record, for it is our view that as the study of the subject develops, increasing numbers of different types of chemicals will be found to display selective herbicidal action, and the recording of data will contribute to the ultimate understanding of the relationships between chemical constitution and herbicidal effect.2-Benzoylbenzoic acid and many of its derivatives are readily accessible chemicals since they are employed in the synthesis of anthraquinone derivatives. Since auxin-like activity, as well as the selective herbicidal effect which it is believed may depend on this biochemical function, is found in aromatic and heterocyclic carboxylic acids in which the carboxylic acid group is attached through a linking group to a cyclic nucleus, 2-benzoylbenzoic acid appeared to be a suitable choice for examination. This substance is considered to have the structure I, but it gives rise to isomeric pairs of esters, the normal esters of configuration corresponding to I, and the pseudo esters corresponding to structure IIe"7. OHThe benzoylbenzoic acid derivatives studied are all described in the literature and were prepared by conventional methods, being characterized by their melting points, which in all cases corresponded with the" recorded data. The esters were of the normal configuration and the pseudo esters were not examined. The preparation of 2:4: 5-trichlorophenyl-acetic acid has been described by S3xton8. The biological methods employed were described by Templeman and Sexton2 and consisted of a laboratory germination test at a concentration of 200 parts per million of test substance in water, and a soil test. The acids were examined as sodium salts and the esters in dispersed form, prepared by dissolving them in a little alcohol and pouring into a large bulk of water. In the soil test, oats and charlock were sown together in boxes in the open, the chemical being applied in dry form diluted with sand at a rate equivalent to 25 lb. per acre. oc-Naphthylacetic acid was used as a standard of comparison.In accordance with the method previously used, the activities of the various compounds against charlock are recorded relative to that of a-naphthyl-acetic acid, as follows: Class A: Substances of greater activity than a-naphthylacetic acid. 2:4: 5-Trichlorophenylacetic acid.Class ?: Substances approximately equal in activity to a-naphthylacetic acid. 2-(4/-Chlorobenzoyl) benzoic acid. Class C: Substances of some activity but less active than a-naphthylacetic acid.2-Benzoylbenzoic acid. 2-(3': 4'-Dichlorobenzoyl) benzoic acid. 2-(4'-Chloro-3'-nitrobenzoyl) benzoic acid. 2-(a-Naphthoyl) benzoic acid. Class D: Substances of low or doubtful activity or inactive. Methyl 2-benzoylbenzoate. Ethyl 2-benzoylbenzoate. 2-Benzoyl-4-(5)-chlorobenzoic acid. 2-Benzoyl-4: 5-dichlorobenzoic add. 2-Benzoyl-3: 4: 5: 6-tetrachlorobenzoic acid. 2-(2': 5'-Dichlorobenzoyl) 3: 6-dichlorobenzoic acid. 2-(2': 5'-Dichlorobenzoyl) 3: 4: 5: 6-tetrachlorobenzoic acid (damage to oats).2-(4/-Chloro-3/-aminobenzoyl) benzoic acid. 2-(4'-Methylbenzoyl>benzoic acid. 2-(4'-Chloro-a-naphthoyl) benzoic acid. Phenylacet?c acid. In the seed germination test, it is known that the susceptibility of charlock and the resistance of oats is a characteristic response to Ÿ-indolylacetic acid, a-naphthylacetic acid and the substituted phenoxyacetic acids. Further, as all these chemical types produce characteristic effects such as epinastic responses on mature plants, it would appear that the mechanism of growth-inhibition of charlock may be concerned with hormonal defunction. 2:4: 5-Tri-chlorophenylacetic acid also fits into this picture, for an epinastic response of oat coleoptiles on emergence was observed, and it is a powerful inhibitor of charlock seed germination. Preliminary experiments have indicated that a response in the Went pea test is given by one of the more active members of the benzoyl -benzoic acid series, namely, 2-(3': 4'-dichlorobenzoyl) benzoic acid; but the generality of the parallelism between the effect on charlock and auxin-like activity cannot be asserted until a more comprehensive examination has been made of these and other compounds. Veldstra9 has put forward the hypothesis that the biochemical effects of auxin-like substances have a physical basis, and that activity is found in substances in which an acidic group is bound to a cyclic nucleus in such a way that the direction of the negative pole lies at an angle (ideally, at right angles) to the plane of the ring. The activity of the benzoylbenzoic acids is not inconsistent with this hypothesis when it is borne in mind that the benzoyl radical can rotate freely about the bond uniting it to the benzoic acid nucleus (? in formula I), so that there is the possibility of the -COOH group projecting at an angle to the plane of the benzene ring (B) of the benzoyl group. It is perhaps significant that the weighting of this particular ring (B) alone by appropriately disposed chlorine atoms does not decrease the activity. Indeed, the activity was definitely increased in on © case. A similar weighting of the ring ? with which the -COOH group is coplanar reduced the activit