For hundreds of years mankind has utilised micro-organisms to produce a whole range of natural products which we can use (e.g. ethanol, organic acids, dextrans, antibiotics). Micro-organisms are extremely easy to cultivate and large scale culture results in high yields of the product required which can then be purified and utilized. Natural products can also be extracted from plant tissue. Biologically active compounds from animals can be isolated from the appropriate organ or tissue but as these are extremely potent compounds, they are normally only present in small quantities and large amounts of the appropriate tissue are required to obtain useful quantities of the product. This is a particular problem with compounds of human origin due to lack of cadavers and to the possibility of contamination of the resulting product with human viruses such as hepatitis and the AIDS virus.