Recently there has been much excitement regarding the use of isoacids as a feed additive. In the fall of 1985, isoacids became available as a new FDAapproved feed additive for dairy cows by Eastman® Chemical Co., under the trade name Eastman IsoPlus Nutritional Supplement. News releases in 1986 reported promising results obtained by Dr. Steven Nissen at Iowa State University in research with one of the isoacids, 2-ketoisocaproic acid. The objective of this paper is to explain what isoacids are, their mode of action, and the results of the early feeding trails of IsoPlus™. Isoacids are the branched ketoacids resulting from the natural rumen degradation of their corresponding amino acids. IsoPlus™ is the calcium salt of four volatile fatty acids (isobutyric, isovaleric, valeric, and 2-methylbutyric). Valeric acid is a straight chain 5-carbon fatty acid. The other three are isoacids (branched-chain fatty acids). Isobutyric, isovaleric and 2-methylbutyric are produced in the rumen mainly by oxidative deamination and decarboxylation of the amino acids valine, leucine, and isoleucine respectivelyl,2,3 (Figure 1). Valeric acid is produced mainly from carbohydrate or from amino acids such as proline.