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Notes: Abstract The members of the work group “Dominant Lethal Mutations of the ad hoc Committee Chemogenetics” jointly carried out experimental studies in the period from November 1972 until February 1976. On the basis of the results obtained and the experience gained, they worked out on February 27, 1976, a standard protocol for the dominant lethal test (DLT) on male mice. The recommendations are: 1. The mating period should preferentially be as short as possible to obtain information about germ cell-stage specific actions of chemical mutagens. Its selection should be based on the objectives of individual investigators. For screening purposes, and with respect to a high fertilization rate, a 4-day mating period is recommended. 2. The total test period should cover at least 12 mating periods, i.e. the whole spermatogenic cycle. A limitation of the DLT to certain “critical” mating periods of high sensitivity is only permissible in repeat tests, or if the parts of gametogenesis concerned, e.g. spermatogoniogenesis or spermatocytogenesis, are studied by cytogenetical tests. 3. The mode of mating should preferentially be 1∶1. 4. The number of test animals to be used depends on various biological and statistical factors; no generally valid statement can be made concerning the number of test animals. By rule of thumb in the order of 50 fertilized females per group per mating interval is recommended. 5. The individual dosing of the substance in terms of mg/kg body weight is to be preferred. Otherwise, the weights of the animals should not deviate more than 5% from the mean value. 6. Results obtained from ill animals or those that died in the course of the trial should not be included in the evaluation. 7. The sensitivity of the mouse strain used against a standard dose of a known mutagen should be regularly checked; this evidence should be presented in publications. 8. For the following test conditions each investigator should make an optimal choice: animal strain, animal age and housing conditions. The investigator has also to decide whether a preliminary mating, in order to check the fertility of the animals to be used, and vaginal-plug evidence are useful in a given case. 9. The autopsy of the dams is best carried out a fortnight from the middle of the mating interval. 10. The results should be documented as completely as possible in the investigational report. The following data were also considered as essential, serving a direct comparison of the test groups: number of mated females (absolute), number of females with implantation (absolute and in %), number of implantations and of live and dead implants (absolute and per female). If the corpora lutea graviditates were counted, their number as well as the pre-implantation loss (absolute and per female) are to be stated. These data are also desirable for publications. 11. For the biological evaluation the following formula can be used for the calculation of dominant lethal factors: FL% = [1-live implants per female of the test group/live implants per female of the control group]×100 12. Statistical evaluation is an essential part of the DLT and various methods are known. For the statistical evaluation it is decisive which biological model and which statistical criteria are most important for the investigator. The DLT must be carried out according to the requirement of the model chosen.