Although adjuvants occur in large quantities in the environment only two products, nonylphenol and 4-nonylphenol, are listed as priority chemicals in the water framework directive. This lack of regulation exists mainly because the applied adjuvants in a pesticide formulation are protected by industry are not disclosed to the public. Consequently, hardly any information on the toxicity, toxicological mode of action and environmental fate is available for authorities and the public. Furthermore, a lot of adjuvants are mixtures of different compounds and cause a lot of analytical challenges. Only very recently, US EPA considered requiring public disclosure of all ingredients of pesticide formulations. Most studies regarding adjuvants focus on the efficacy and only few research papers focus on toxicity and environmental fate. Nevertheless, there is an urgent need for information concerning the toxic mode of action, residue levels and the environmental fate of adjuvants for correct risk assessment and estimation of threshold levels. Information on the toxic mode of action of compounds is important to develop a solid scientific basis for risk assessment. The use of appropriate alternative in vitro systems, can provide relevant information to facilitate regulatory decisionmaking. Moreover, the use of non-animal tests is promoted by the new EU crop protection regulation. The European OSIRIS project, proposes that a good way to improve the evaluation of chemicals may be by categorisation in modes of toxic action. In this way, priorities for the evaluation of compounds can be set based on the toxic modes of action like for example the genotoxic potential of a compound. The in vitro assay used in this study is an example of such a test system. The multiple endpoint bacterial reporter assay is based on the induction of specific signalling pathways that are universal in the living cell and hence the assay is able to combine the detection of toxic compounds and at the same time provide information on a number of universal mechanisms of toxicity. In the present study we applied the bacterial multiple endpoint reporter assay to evaluate different adjuvants at the toxicity and toxic mode of action level. In a first step, bacterial growth inhibition is quantified and compared between the different adjuvants. Secondly, new information regarding different mechanisms of toxic action, i.e. DNA damage, oxidative stress, membrane damage and general cell lesions is obtained and these results are applied to categorise the adjuvants according to the mechanisms of toxic action. The toxicological results of this study are applied to select adjuvants that have a preference of use. Adjuvants comprise of three major groups: surfactants, high content screening solvents and synergists and are often referred to as “inert ingredients”. A consumer survey performed by US EPA learned that many consumers are mislead by the term “inert ingredient”, believing it to mean harmless. This certainly is not the case and in fact they can be toxic to humans, may have biological activity of its own.