The up-and-down method is one of three new sequential methods recently accepted for use by all OECD countries. The fixed dose method involves dosing groups of five animals in a sequential fashion at a choice from five fixed doses. This method incorporates a sighting sequence and uses signs of toxicity—not death—as an endpoint. The acute toxic class method also uses a choice from the same five fixed doses, but instead of dosing five animals at each step, this method tests either three or six animals at each dose. The acute toxic class method uses death as the end-point. As recently updated and revised, the fixed dose procedure and the acute toxic class method are designed specifically to allow classification of new materials according to the recently developed Globally Harmonized Classification System (OECD 2001b); they do not provide a point estimate of LD50. The UDP used does not take advantage of the information available on the sequence of events; it uses only the final results. An analysis that reflected the path taken to reach the results would also include the dependencies of dose choices and would be much more complicated. In the future, it may be possible to find a way to use all of the information in a way that will improve the accuracy of the test.