When checking in in Europe, if you let UA know you are continuing on--especially if you ahve 2 seperate tix, it is possible to have your bag tagged all the way through to EZE. However, because at IAD you are actualyl "entering" the United States--there is no seperate "Transit" longue for Int'l-Int'l connections--you will ahve to claim your bags and clear US customs. lately when i have come in on the 740pm LHR-IAD flight the mid-field terminal Customs/Immigration line has been open because UA times the late bank of US departures from IAD to hook up with those late LHR and FRA flights. Presumedely you should be able to get your UA boarding passes for all your flights and have your luggage tagged all the way through. So you would only have to clear the US formalities and then take the escalator up and go right to your EZE departure gate and board the plane. but the important thing is that this all take place right in the mid-field terminal. If I am wrong and somehow you had to bus to the main terminal, clear, and then return to mid-field then you could be cutting it short. I have had terrible times with luggage and clearance on the late flights at main terminal becasue UA is no the only carrier that has flights now arriving 7-8pm and UA baggage seems to want to move all the transfer bags to their departing before beinging bags over to the main terminal for those who are ending travel at IAD. But it is very important to remember, if your bags are tagged as connecting from IAD to another city--and mid-field customs/immigration station is operating---your bags will go to the mid-field transfer baggage claim and not to the main terminal.