MacConkey agar is a culture medium designed to grow Gram-negative bacteria and stain them for lactose fermentation. It contains bile salts, crystal violet dye (to inhibit Gram-positive bacteria), neutral red dye (which stains microbes fermenting lactose), lactose and peptone. This means that MacConkey agar is both selective and differential. Alfred Theodore MacConkey developed it while working as a bacteriologist for the Royal Commission on Sewage Disposal.

Because MacConkey agar selects for enteric pathogens, it is used in the diagnosis of diarrhea. The important pathogens are Gram-negative organisms such as Shigella and Salmonella.

A variant, MacConkey Sorbitol Agar, permits the isolation and differentiation of enteropathogenic E. coli serotypes.