Experimental control material for statistical analysis of the results of the micronuclei test in the mouse (NMRI strain) and the Chinese hamster and for the host-mediated assay in the mouse (NMRI strain) using auxotrophic bacterial strains are presented. The binomial distribution of the micronuclei makes it possible to analyse the sample size according to the formula of Cochran and Cox (1957). For the host-mediated assay, the experimental principles are given which make it possible to evaluate the results obtained even with a weakly mutagenic, unknown substance. Critical points in comparative tests are not only the methodological questions, but also pharmacokinetic problems of the substance being tested which can only be clarified in the species used for the mutagenicity test. If this is ignored then even experimentally based findings can only be recorded as speculations.