Microorganisms play an important role in the development of the chemistry of natural products and medical therapy. Microbial metabolites are among the most important of cancer chemotherapeutic agents. A number of microbial compounds capable of suppressing the immune response have been discovered. Further applications are possible in various areas of pharmacology, developments catalyzed by the use of simple enzyme assays for screening before testing in intact animals. Microorganisms have been considered to be a rich source of unique bioactive compounds. Many microbial-originated antibiotics are available in the market, and more than 120 of the most important medicines in use are obtained from terrestrial microorganisms.