We demonstrate a steep negative hyperbolic relationship between the specific respiration activity of soil microorganisms (expressed as production of CO2-C per unit microbial biomass and unit time) and their microbial biomass. Activity drops by two orders of magnitude when microbial biomass increases from very low values of 7-mu-g C g-1 dry soil to about 0.4 mg and then remains nearly constant irrespective of biomass. The same relationship was found to be valid for original data from fallow, field, meadow and forest soils at a locality in Southern Bohemia and also a set of "world" data collected from the literature. The existence of a similar relationship in aquatic bacteria as well as in laboratory cultures of fungi and bacteria is pointed out. The statistical significance of the relationship is evaluated from the "spurious" correlations point of view. Four hypotheses for the possible cause of this dependence are outlined. Inhibition of the specific respiration activity by the CO2 produced metabolically seems probable.