Composite agar with glucose, lactose and ferric citrate with phenol red indicator, to determine the ability of enterobacteria to ferment one or both of the carbohydrates and to produce H2S. The medium is tubed as a slant with butt base; both are inoculated. If both show acidification, lactose is fermented; if only the butt shows acidification (yellow) only glucose is fermented. Gas is seen as bubbles, H2S causes blackening of the medium.