a yellow, water-soluble organic compound that occurs abundantly in whey (the watery part of milk) and in egg white. An essential nutrient for animals, it can be synthesized by green plants and by most bacteria and fungi. The greenish yellow fluorescence of whey and egg white is caused by the presence of riboflavin, which was isolated in pure form in 1933 and was first synthesized in 1935. It has the following chemical structure:

Riboflavin functions as part of metabolic systems concerned with the oxidation of carbohydrates and amino acids, the constituents of proteins. Like thiamin (vitamin B₁), it is active not in the free form but in more complex compounds known as coenzymes, such as flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD), or flavoprotein. Riboflavin is widely distributed in both plants and animals, but its abundance varies considerably. Milk, eggs, leafy vegetables, kidney, and liver are good dietary sources. An adult human needs 1.0 to 1.3 mg (1 mg = 0.001 gram) of the vitamin per day.

A dietary lack of riboflavin is characterized by variable symptoms that may include reddening of the lips with cracks at the corners of the mouth (cheilosis); inflammation of the tongue ( glossitis); ocular disturbances, such as vascularization of the eyeball with eyestrain and abnormal intolerance of light; and a greasy, scaly inflammation of the skin. Some disagreement persists as to the characteristic syndrome of riboflavin deficiency in humans because it tends to be associated with a deficiency of other vitamins, notably niacin (see pellagra).