马康基氏琼脂（英文：MacConkey Agar）为一种细菌繁殖营养成分，但只对革兰氏阴性菌有效，而且由于它的特殊成分(NEUTRAL RED指示剂)，可以辨别会利用乳糖发酵和无法利用乳糖发酵的细菌。

会利用乳糖发酵的细菌在马康基氏琼脂上的颜色为红色，而无法利用乳糖的细菌则呈现黄色。（如图）

主要颜色差异则在于微生物发酵乳糖产生酸，会导致酸碱值的差异性，例如大肠杆菌这种可以利用乳糖发酵的细菌在马康基氏琼脂上就会是红色，而他类如 Salmonella 则是黄色。

MacConkey agar is a selective and differential culture medium for bacteria designed to selectively isolate Gram-negative and enteric (normally found in the intestinal tract) bacilli and differentiate them based on lactose fermentation. The crystal violet and bile salts inhibit the growth of gram-positive organisms which allows for the selection and isolation of gram-negative bacteria. Enteric bacteria that have the ability to ferment lactose can be detected using the carbohydrate lactose, and the pH indicator neutral red.