1st Regiment, Illinois Cavalry
Seven Companies, "A" to "G," organized at Alton, Illinois, and mustered in July 3, 1861. The Regiment mustered out July 14, 1862.
Companies "I," "H" and "K" were not mustered with Regiment and never served with it. These companies mustered out December 27, 1862. 
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.
Seven Companies, "A" to "G," organized at Alton, Illinois, and mustered in July 3, 1861.
Company "H" was organized at Alton, Illinois, June 12, 1861.
Company "I" organized at Alton, Illinois, July 9, 1861.
Company "K" organized at Alton, Illinois, December 9, 1861.
Other Source Material
- Illinois Civil War Rosters from the Adjutant General's Report, 1st Regiment, Illinois Cavalry (accessed 10 May 2011) includes rosters of all the regiments and information on the different companies in each regiment.
- National Park Service, The Civil War Soldiers and Sailors System, (accessed 15 January 2011).