The Toxicology Unit was not conceived in response to a particular environmental or industrial problem, but was due to the foresight of a few people, who appreciated in the 1940's that the development of industry, pharmaceutics and medicine in the post-war period would expose large populations to new types and classes of potentially hazardous chemicals. The Unit was originally established at Porton Down in 1947 under the Directorship of John Barnes with Norman Aldridge as the only member of staff. In 1950 the Unit moved to the Carshalton site and flourished in the following years reaching the forefront of research in toxicology. This was mainly the result of a pioneering approach: to study the mechanism of action of chemicals at all levels from the animal to the individual molecules. It was believed that this approach would facilitate the understanding of the toxic effects of classes of compounds and possibly would permit generalisations to be made about mechanisms of toxicity. The success of this approach was reflected in the leading role that the Unit immediately acquired worldwide, becoming the inspiration for the creation of similar research establishments in other countries. The multidisciplinary approach and the goal to investigate fundamental mechanisms proved to be successful over the subsequent decades and led to the understanding of the toxicity of environmental pollutants including pesticides and solvents.