Source Naturals Acidophilus is a specially cultured strain of viable, freeze-dried Lactobacillus acidophilus. The body relies on "friendly flora" residing in the intestines for the internal protection against putrifactive yeasts and bacteria, which can secrete toxins and hoard vital nutrients. Acidophilus supports colon health by altering the microflora ecology and favoring "friendly" flora. It also manufactures some B-vitamins, especially folic acid.