Abstract: A binary assay identifies agents that inhibit sterol .DELTA.14 reductase involved in ergosterol biosynthesis. In the primary screen, sterol .DELTA.14 reductase inhibition by a test sample is assayed by adding the test sample to a culture of Neurospora crassa having an erg-3 mutation and also to a culture of a strain having an erg-1 mutation, comparing the extent of growth inhibition after incubation in the two cultures, and identifying as positives those samples that show growth inhibition in the erg-3 culture exceeding that in the erg-1 culture. In the secondary screen, samples that test positive in the primary screen are reassayed by adding the test sample to a culture of a Saccharomyces cerevisiae strain into which has been introduced multiple copies of a gene encoding sterol .DELTA.14 reductase and also to a strain of S.