The potentialities of the quantitative dehydrogenase method in the analysis of problems of both agricultural soils and deposits underlying natural waters are discussed. The method may be used to determine: actual microbial activity; potential microbial activity; the effects of toxic and inhibitory substances on microbial activity; and the relative physiological availability of nutrients in soils and bottom deposits of natural waters. The need for the standardization of the method for the various purposes is stressed and sources of possible error are mentioned.