Physician credited in a 2011 account with the 1919 report of the effect of mustard gas on the blood
Edward B. Krumbhaar
A 2011 history says Edward Krumbhaar treated soldiers exposed to mustard gas at a hospital in France and in 1919 reported a change in their blood counts, an early rise in red and white cells followed by a marked fall in circulating white cells. The same account links this observation to the later Yale work on nitrogen mustard.
Sources
Christakis P. The birth of chemotherapy at Yale. Bicentennial lecture series: Surgery Grand Round. Yale J Biol Med. 2011;84(2):169-72.