21st Regiment, Arkansas Infantry (Confederate)
21st Infantry Regiment was organized during the winter of 1861-1862 by consolidating four companies of J. S. McCarver's 14th and six companies of G. W. Lomoyne's 17th Arkansas Battalions. The regiment was involved in the Battles of Corinth and Hatchie Bridge, and reported 27 killed, 41 wounded, and 58 missing. It was then assigned to General M. E. Green's Brigade, Department of Mississippi and East Louisiana. The 21st sustained 37 casualties at Port Gibson and was captured at Vicksburg on July 4, 1863. It was not reorganized after the exchange. 
See also: EDWARD G. GERDES, Arkansas Civil War Regiments, Rosters and Muster Rolls, 
Companies in this Regiment with the Counties of Origin
Men often enlisted in a company recruited in the counties where they lived though not always. After many battles, companies might be combined because so many men were killed or wounded. However if you are unsure which company your ancestor was in, try the company recruited in his county first.
- Beginning United States Civil War Research gives steps for finding information about a Civil War soldier or sailor. It covers the major records that should be used. Additional records are described in 'Arkansas in the Civil War' and 'United States Civil War, 1861 to 1865' (see below).
- National Park Service, The Civil War Soldiers and Sailors System, is searchable by soldier's name and state. It contains basic facts about soldiers on both sides of the Civil War, a list of regiments, descriptions of significant battles, sources of the information, and suggestions for where to find additional information.
- Arkansas in the Civil War describes many Confederate and Union sources, specifically for Arkansas, and how to find them.. These include compiled service records, pension records, rosters, cemetery records, Internet databases, published books, etc.
- United States Civil War, 1861 to 1865 describes and explains United States and Confederate States records, rather than state records, and how to find them. These include veterans’ censuses, compiled service records, pension records, rosters, cemetery records, Internet databases, published books, etc.