Calculation of the EC50 and its confidence interval when subtoxic stimulus is present

In many experiments in toxicology and ecotoxicology with continuous response, like growth, a stimulus is found for low doses, while higher doses are toxic. This subtoxic stimulus is called hormesis. The standard logistic model cannot be used when hormesis occurs. A model is given which can describe this phenomenon. The model has the EC50 as one of its parameters. This has the advantage that the EC50 and its confidence interval can be determined directly by nonlinear regression. Also, the tests for the presence of hormesis and lack of fit are described.