For those with ancestors who served in Massachusetts regiments, an absolutely vital resource is located at the Massachusetts National Guard Museum and Archives, in Worcester:
http://www.civilwarnews.com/archive/articles/mass_records.htm
Besides a microfilm copy of the complete Mass. Service Records from the National Archives (no need to order from D.C.), they also have the following series of Civil War documents, all available on site and free of charge:
Draft Registrations (by town), 1861-1865, 21 boxes
Rosters of Officers, 1861-1865, 4 vols.
Massachusetts Volunteer Militia, Muster and Descriptive Rolls, 1861-1865, 69 vols.
Oaths of Officers, 9 Month Regiments, 1861- 1865, 2 vols.
Muster-in Rolls, Massachusetts Officers, 1861- 1865, 7 vols.
General & Special Orders, 1861-1865 5 vols. Paroled Prisoners & Certificates of Death, 1861- 1865, 1 vol.
Deaths in Rebel Prisons, 1861-1865, 3 vols.
Long Island, Boston Harbor, Muster and Descriptive Rolls, 1863-1865, 1 vol.
Civil War Atlas, 1861-1865 3 vols. Character of Returned Soldiers, 1861-1865, 1 vol.
Copies of Discharges, 1861-1865, 9 vols.
Desertions & Discharges, Massachusetts Volunteers, 1861-1865, 13 vols.
Returns of Recruits Examined, 1861-1865, 7 vols.
Enrollments and Medical Examinations by Town, 1861-1865, 4 vols.
Surgeon General Medical Documents, 1861-1865, 6 boxes
Notices of Discharge, 1861-1865, 16 vols.
Civil War Monthly Reports, 1861-1865, 15 vols.
Hospital Records by Location, 1861-1865, 4 vols.
Medical Certificates for Extension of Furloughs, 1861-1865, 4 vols.
Hospital Records, 1861-1865, 2 vols.
Deaths in Rebel Prisons and Exchanged Prisoners, 1 vol.
Arsenal Records, 1861-1862; 1864-1865, 2 boxes
Hospital Records, 1861-1865, 1 box
Medical Papers, Sick and Wounded Reports, and Hospital Supplies, 1861-1865, 1 box
Medical Letters, 1861-1862 (Vol. 3), 1 box
Hospital Stewards' Credits for Bounty, 1864 1 vol.
Bounty Numbers, U.S. Colored Troops and Enlistments in other States, 1861-1865, 9 vols.
Descriptive Rolls by Regiment, 1861-1865, 39 vols.
Civil War Regiments of the Massachusetts Volunteer Infantry, 1861-1865, 125 boxes
Civil War Enlistments by Regiment, 1861-1865, 84 boxes
Surgeon General Correspondence, 1861-1865, 28 boxes
Civil War Personal Collections, 1861-1890, 5 boxes
Massachusetts Regimental Histories, 1861- 1865, 126 vols.
Enlistment Papers, 1863-1864, 46 vols.
Certificates of Disability for Discharge, 1861- 1865, 8 vols.
Governor Andrews Letters, 1861-1866, 1 box
Executive Department Correspondence Letters Sent, 1861-1866, 25 vols.
Orders from the Governor, 1861-1875, 18 vols.
Bounty Numbers, 1861-1865, 50 vols.
Surgeon General Correspondence, Massachusetts Medical Officers, 1861-1862, 1 box
Surgeon General Correspondence, Military Officers' Letters, 1861-1867, 8 vols.
U.S. Medical Officers Letters, 1861-1865, 2 vols.
Massachusetts Medical Commission Letters, 1861-1865, 3 vols.
Deaths of Massachusetts Soldiers in Various States, 1861-1865 1 vol.
Soldiers' Home Records, Chelsea 3 boxes Regimental and Hospital Medical Reports, 1861- 1865, 1 vol.
Hospital Record Register, 1863-1866, 2 vols.
There are also many records regarding Massachusetts' participation in earlier and later wars.
The Mass. National Guard includes the oldest existing military formations in the U.S. The 181st and 182nd Infantry Regiments, the 101st Engineer Battalion, and the 101st Field Artillery Regiments, all originate in the North, South, East, and West Regiments created by the Mass. General Court on 13 December 1636 -- which is also the official "birthday" of the National Guard of the United States. The 181 and 182 Infantry Regiments are the only units to display the Lexington/Concord battle streamer on their colors.
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