Research was conducted toward development of a sensitive, biologically nonspecific assay technique which uses the firefly bioluminescent reaction to detect ATP extracted from microorganisms present in water supplies. An instrument using a photomultiplier to detect the light output from the bioluminescent reaction was designed, constructed, and tested. Cultures of a wide variety of organisms were grown, both heterotrophic and autotrophic, and included numerous species of bacteria, algae, fungi, and protozoa. The tests demonstrated the ability to detect concentrations of only a few hundred cells of any of the test species and in many cases less than 100 cells in less than 30 seconds. The ATP response is related to number of cells and cell size and can be used as a measure of the number of cells per volume of sample if something is known of the cell size. Development of a differential filtering technique may be necessary to obtain highest accuracy in quantitation if cells of varying size are encountered. The inherent light level of the enzyme solution without ATP added varies from day to day and consequently the net response of the instrument - gross response less inherent light - is the best measure of ATP level. The assay technique may have applicability to terrestrial water supplies - particularly under hazardous or difficult conditions - as well as to spacecraft waters.