As always, casualty figures vary, but here is some anecdotal data from the Iron Brigade at South Mountainb. Sidney B. Mead, a member of F/2 WIS, kept a journal, the MSS of which is at the Wisconsin Historical Society. Hisentry for Sep 14 states "the casualties in this battle were of the 2nd [Wis] in killed and wounded 27 -- of the brigade total 309."
George H. Otis, of I/2 WIS states the "brigade lost in killed & wounded 309. The 2nd Wis. lost 9 killed, 18 wounded. ....." His Diary is also at the Wisconsin Historical Society. George B. Ely, a non-commissioned officer in the 2 WIS, wrote to his father on 21 Sep 62, speaking of South Mountain, saying, "In this engagement our Brigade suffered a loss of over 400." Edwin Bentley Quiner Collection, Correspondence of the Wisconbsin Volunteers, Wiisconsin Historical Society.

On the other hand, an unidentified contemporary newspaper clipping in the same Quiner Collection states that "This brigade lost about 120 killed and wounded."

Another piece of correspondence in the Quiner file from Capt. John B. Callis, who commanded the 7 WIS at South Mountain, states that "the aggregate of killed, wounded and missing [in the 7 WIS] was about 147; the regiment went into action with 375 muskets."

Whatever had been the losses at Soth Mountain, it is celar that the Iron Brigade had lost heavily. Franklin A. Hasskell, a member of the brigade, wrote that the losses of the Brigade at Sharpsburg were in killed and wounded 380, 47.5% of the number engaged, which would make the entire Iron Brigade at Antietam less than 800 strong. Frankklin Artes Haskell, Haskell of Gettysburg: His Life and Civil War Papers, published 1970 by the Wisc. Historical Society.

Similarly, George W. Partridge, of the 19 IND, wrote on 20 Sep to his sisters that "this Regt [19 IND] [after Antietam] now numbers about 150. the 2 [WIS] Regt does not number 100 now." Hugh L. Whitehouse, ed., Letters from the Iron Brigade: George Washington Partridge ..., 1994, Guild Press.

Much of the decimation of the units of the Iron Brigade had come at South Mountain. It was a unit of thoroughbred fighters.