Everyone involved in planning and carrying out biological experiments has to concern himself with statistical methods. Thus, descriptive statistics (e. g. calculation of the mean) help to present results briefly and comprehensively and, in addition, statistical test procedures permit decisions to be made even in an uncertain situation. The choice of the appropriate statistical procedure is, however, associated with considerable problems. These problems are discussed in detail using the dominant lethal test as an example. Means of solving the problems with the necessary consequences for planning of the experiment and its evaluation are discussed and practical recommendations are given applying also to other mutagenicity tests.