A method and device for detecting bacteria by forming an immunoassay laddersing a biological substance (Ag) adsorbed onto a surface, to which has been added an immunological biochemical ladder (Ab) and beta-galactosidase enzyme (gal) to form a Ag-Ab-gal system. The system is added to an aqueous solution of an enzyme selected from ortho-nitrophenylgalactopyranoside and ortho-nitrophenylgalactosidase enzyme. The enzyme reacts with the gal to produce ortho-nitrophenol (ONP) such that the amount of ortho-nitrophenol (ONP) produced may be detected by measuring the vapor pressure generated by the ONP using an ion mobility spectrometer monitor. The pressure of the ONP is in proportion to the amount of biological substance present in the system. In a preferred embodiment, the ion mobility spectrometer monitor includes display means for indicating the quantity of bacteria measured. The surface on which the system is formed is preferably a plastic surface positioned in a cylindrical container sized to engage either an Airborne Vapor Monitor or a Chemical Agent Monitor, either of which may be hand held.