Yeast Extract is the water-soluble portion of autolyzed yeast. The autolysis is carefully controlled to preserve naturally occurring B-complex vitamins. Yeast Extract is prepared and standardized for bacteriological use and cell cultures and is an excellent stimulator of bacterial growth. Yeast Extract is one of the most complete and versatile fermentation supplements available. It is an important ingredient for the microbiological assay of vitamins. Yeast extract is also of value in the assay of antibiotics. B factor, a growth substance necessary for the production of rifampin in a Nocardia species, can be isolated from yeast extract.