Generally, mushroom contains 85-90 per cent water of its dry matter. However, amount of water is greatly influenced by relative humidity and temperature during growth and storage. Protein is the most critical component which contributes to a lot of nutritional value of food. Not all, only 34-89 per cent mushroom proteins are digestible. Amount of protein varies from 34 per cent to 44 per cent of total dry weight in Agaricus sp. The crude fat content ranges from 1-20 per cent of total dry weight. Besides protein, a large variety of free and combined fatty acids also occur in A. bisporus with high concentration of palmic acid, stearic acid and oleic acid. Fresh mushroom contains relatively large amount of carbohydrates i.e. 3-28 per cent, particularly pentoses, hexoses, disaccharides, and trehlose (a mushroom sugar). They appear as a good source of several vitamins (thiamin, riboflavin, niacin, biotin, ascorbic acid, vitamin A, B, C, D, and minerals (sodium, potassium, calcium, iron, etc.), essential amino acids (methionin, citralline, ornithin) and several undesirable elements (cadmium, selenium, chromium, etc.).