The amount of enzyme present or used in a process is difficult to determine in absolute terms (e.g. grams), as its purity is often low and a proportion may be in an inactive, or partially active, state. More relevant parameters are the activity of the enzyme preparation and the activities of any contaminating enzymes. These activities are usually measured in terms of the activity unit (U) which is defined as the amount which will catalyse the transformation of 1 micromole of the substrate per minute under standard conditions. Typically, this represents 10-6 - 10-11 Kg for pure enzymes and 10-4 - 10-7 Kg for industrial enzyme preparations. Another unit of enzyme activity has been recommended. This is the katal (kat) which is defined as the amount which will catalyse the transformation of one mole of substance per second (1 kat = 60 000 000 U). It is an impracticable unit and has not yet received widespread acceptance. Sometimes non-standard activity units are used, such as Soxhet, Anson and Kilo Novo units, which are based on physical changes such as lowering viscosity and supposedly better understood by industry. Rightfully, such units are gradually falling into disuse. The activity is a measure of enzyme content that is clearly of major interest when the enzyme is to be used in a process. For this reason, enzymes are usually marketed in terms of activity rather than weight. The specific activity (e.g. U Kg-1) is a parameter of interest, some utility as an index of purity but lesser importance. There is a major problem with these definitions of activity; the rather vague notion of "standard conditions". These are meant to refer to optimal conditions, especially with regard to pH, ionic strength, temperature, substrate concentration and the presence and concentration of cofactors and coenzymes. However, these so-termed optimal conditions vary both between laboratories and between suppliers. They also depend on the particular application in which the enzyme is to be used. Additionally, preparations of the same notional specific activity may differ with respect to stability and be capable of very different total catalytic productivity (this is the total substrate converted to product during the lifetime of the catalyst, under specified conditions). Conditions for maximum initial activity are not necessarily those for maximum stability. Great care has to be taken over the consideration of these factors when the most efficient catalyst for a particular purpose is to be chosen.