An improved method for detecting very low concentrations of luminescent reactive molecules from microbial cells in samples of fluids and degraded solids is disclosed. A sample containing microbial cells is treated to eliminate substantially all nonmicrobial material. The microbial cells are then caused to rupture and form a thin film positioned in the vicinity of a photodetector. An appropriate reagent contacts the thin film and the resulting photon emission is observed by the photodetector through an optical filter means which selectively restricts the wavelength of photons which are permitted to pass from the sample to the detector. The strength of the luminescence within the permitted wavelength is measured and recorded as an indication of the concentration of microbial cells in the sample.