We can distinguish between so-called 'all or none' responses,such as death,and graded responses,such as the inhibition of an enzyme or the level of a marker of pathological damage.Both 'all or none' responses and graded responses can show a typical dose-response relation. In both cases there will be a dose at which there is no measurable effect and an upper dose where there is a maximal responses. very often in a toxicity study, either in whole animals or in isolated cells, lethality will be the first parameter of toxicity utilized bot this gives little if any information about the underlying mechanism of toxicity.However,it is often important to know the limits of dosing in practical terms.Although it is not always necessary to know the lethal dose,it is important to know whether toxicity occurs at the dose or a multiple of the dose likely to be encountered by man or animals.However in certain situations it is extremely difficult or impossible to quantify the likely human dose and may be similarly difficult to extrapolate the likely effects in man form the available data (see also chapter 12).