A method for rapidly detecting and enumerating microorganisms, having a level of microbial ATP, in the presence of mammalian cell preparations, comprising the steps of: providing a mammalian cell preparation comprising mammalian cells having a level of mammalian ATP; reducing the level of mammalian ATP in said cell preparation by, selectively lysing the mammalian cells and not lysing microbial cells with detergent or osmotic shock to extract the mammalian ATP, treating the extracted mammalian ATP with one or more ATP hydrolyzing compounds; and immobilizing the microorganisms and washing away said detergent and the hydrolyzing compound by filtering the mammalian cell preparation through a micropartioned hydrophilic/hydrophobic membrane; extracting the microbial ATP using an extracting reagent; drying the membrane; applying a bioluminescent reagent onto the membrane; and detecting and enumerating the microorganisms.