A diagnostic test system for detecting beta -galactosidase producing micro-organisms is prepared by impregnating one zone of a bibulous material with an aqueous acetone solution of an o-nitrophenyl- beta -D-galactopyranoside substrate and evaporating to dryness; and impregnating a contiguous and separate zone of the bibulous material with a solution of a microbiologically inert buffer system which will maintain a pH of from about 6.5 to 8.5 and evaporating to dryness. Preferably, the bibulous material contains 1.5 mg of dry o-nitrophenyl- beta -D-galactopyranoside and 0.02 millimoles of a buffer which will maintain a pH of 7.4. Optionally, a waterproof barrier zone and an identifying dye zone may be present. In use, the impregnated bibulous material is inserted into a test tube of a saline suspension of unknown organism and incubated for 4 hours. The development of a yellow coloration indicates that beta -galactosidase is produced by the organism being tested. The diagnostic test system of this invention is stable for 1 year at 4 DEG C., at room temperature, and at 37 DEG C.
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