Abstract A number of response variables can be derived from growth inhibition toxicity experiments with microorganisms; currently, there is a controversy regarding which variable to specify in standardized test protocols with algae. Yield or biomass at a specified time is the variable used most frequently. Specific average growth rate is a debated alternative, and appreciable differences may exist between EC figures derived from the same set of data using either test endpoint. The problem is reviewed and analysed mathematically. It is concluded that from a theoretical point of view, growth rate is a better response variable than biomass, e.g. because EC figures estimated from growth rates are less dependent on particular test system parameters. However, as the use of both methods to analyse data will probably continue, it is important to be aware of the potential differences between estimated EC figures.