crystalline cellulose.Check the facebook page Save Natural Thyroid.Wood pulp is another source for microcrystalline cellulose: <a href="http://en.wikipedia....wiki/BorregaardPlant-derived">http://en.wikipedia....wiki/BorregaardPlant-derived</a> cellulose is usually contaminated with hemicellulose, lignin, pectin and other substances, while microbial cellulose is quite pure, has a much higher water content, and consists of long chains. sounds like it is not so