The acute toxic class method (ATC method) is an alternative to the LD50 test, with the use of substantially fewer animals needed for the classification of substances. Like the classical LD50 test the biometry of the ATC method is based on the probit model. The biometric calculations of the ATC method were carried out not only for the classification categories of the European Union but also for the classification criteria of various countries and organizations, currently in use. It is demonstrated that in comparison with the LD50 test in general the same classification is obtained with the ATC method and with the use of substantially fewer animals. Substances with high slopes are likely to be allocated to the predicted toxicity class in comparison with substances having low slopes, with both the ATC method and the LD50 test. Substances are more likely to be allocated into a lower toxicity class with the LD50 test than with the ATC method.