There are considerable differences of opinion as to the most desirable temperature at which activities of enzymes in plasma should be measured. Debate centers around 25 degrees C because of its widespread use in physico-chemical measurements, 30 degrees C because of its supposed technical convenience, and 37 degrees because of its physiological significance. There is, however, widespread agreement that the activities of intracellular enzymes, and of enzymes having a natural function in plasma, be measured at 37 degrees C. Perhaps because estimations of the activities of those enzymes in plasma that have leaked from damaged tissues have been subject to almost continuous modifications, temperature has simply become just another variable to take into account. In this article I describe the relevance of a temperature near 37 degrees C to the activities of all enzymes in man, intracellular and extracellular. I conclude that 37 degrees C is to be favored as the temperature for assaying all parameters in human biochemistry affected by temperature. These include not only enzyme activities but also pH, blood-gas equilibria, binding data, and, in fact, all kinetic activities in the body.