Once a dose-response relationship has been demonstrated there are several parameters which can be derived from it.When lethality has been used as an endpoint,the LD50 can be determined (Figure 1.5).This is defined as the dosage of a substance which kills 50 per cent of the animals in a particular group,usually determined in an acute,single exposure study.It is not an exact value and in recent years there has been much discussion as to its usefulness and necessity in toxicology (see Chapter 12).The LD50 value may vary for the same compound between different groups of the same species of animal.The value itself is only of real use in a comparative sense,giving the toxicologist an idea of how toxic a compound is relative to other substances (Table 1.1,Figure 1.6) or enabling toxicity to be compared using various routes of administration (Table 1.2) or in different species for example (Table 1.3). It is also widely used for classification purposes.