Indole, α‐ and β‐naphtol are synergistic with auxins in enhancing root production of cuttings of Phaseolus vulgaris. To find an explanation for this synergism the activity was compared with that of phenolic compounds which are known to be: a. inhibitors of the IAA oxidase system, b. cofactors of the conversion of tryptophane to IAA. Both processes appeared to occur in the bean cuttings (as estimated by root production), but none of the phenols bad an activity equally strong as the “synergists”. It is suggested that indole as well as α‐ and β‐naphtol act as other injurious substances in “just sublethal” dose (Soekarjo 1966). Copyright © 1969, Wiley Blackwell. All rights reserved