Blood Brotherhood is an  unmarked and repeatable quest in the Fallout 3 add-on .
After the completion of
Who Dares Wins, if you did not destroy the Citadel, Paladin Tristan asks you to kill super mutants and bring back super mutant blood samples so the Brotherhood can research a weapon that could destroy super mutants with ease. These samples can be found on any super mutant you kill, much like ears and fingers on corpses, and can be exchanged with Paladin Tristan in the Citadel's A ring for caps and XP.
50 caps and 10 XP per blood sample.
This is one of the most effective ways to gain XP and caps.
Unlike the ears and fingers found on corpses through perks, super mutants killed by any means after this quest starts will have blood samples on their corpses.
Within D.C. proper, areas that are highly populated with super mutants are The Mall, and the area between Bailey's Crossroads and the Outcast Outpost. Outside the D.C area, Roosevelt Academy, Mason Dixon Salvage, and Vault 87.
Waiting to start the Reilly's Rangers quest until after this quest starts can be highly profitable.
For some reason, Fawkes does not yield a blood sample. This is most likely due to the fact that the game considers him a Companion rather than a super mutant, which allows the NPCs to do the same.
The super mutant behemoths do not produce blood samples.
xbox360 Occasionally, Tristan's dialogue for exchanging the blood samples for caps will go normally, but neither blood samples nor caps will be transferred. This can be solved by not speeding through Tristan's dialogue during the actual transaction.
Fallout 3 Official Game Guide