Assays for compounds having a desired biological activity against one but not both of a pair of microorganism species utilize separately detectable labels. The first and second microorganisms are different from each other, the first microorganisms exhibit a first detectable label, the second microorganisms exhibit a second detectable label different from the first detectable label, and neither of the labels interferes with detection of the other label. In various embodiments, the first and second microorganisms are incubated with a candidate compound or a producer thereof. The different labels permit isolation of microorganisms exhibiting one of the labels but not the other, indicating the desired activity, and the compound responsible for this differential response is isolated.