Session 4, February 14, 2002

Definition of a toxicant and assumptions in dose response

The empirical definition of a toxicant can be constructively viewed as a parallel phrasing of Koch's 1890 postulates regarding the empirical definition of a pathogen.

1.	The specific organism should be shown to be present in all cases of animals

suffering from a specific disease but should not be found in healthy animals.

2.	The specific microorganism should be isolated from the diseased animal

and grown in pure culture on artificial laboratory media.

3.	This freshly isolated microorganism, when inoculated into a healthy

laboratory animal, should cause the same disease seen in the original animal.

4.	The microorganism should be reisolated in pure culture from the

experimental infection.

Note that virtually the same empirical definition can be applied to hormones by substitution of "hormone" for "specific organism," by substituting "normality" for "suffering from a specific disease," by substituting