Abstract: A method is provided for detecting a microorganism which produces a desired substance. The method involves overlaying a membrane on an agar surface in a predetermined orientation. Microorganisms are grown on the membrane and the substance allowed to pass into the agar. The membrane is removed and a detectable reaction between the substance and one or more reagents is observed. The location of the microorganism to be detected can be determined by the predetermined orientation, and the microorganism can be isolated by removing it from the membrane. When the substance is not secreted by the microorganism, the microorganisms are lysed releasing the substance for passage into the agar. The microorganisms producing the substance can be located on replicate plates and isolated.