The theory of Four Great Ancient Civilizations (Mesopotamia, Egypt, China, and India), which is referred to by Madlax and others when researching the Elies language, is actually a real theory developed by Chinese historians.
In Madlax red shoes are important to the past events that happened 12 years ago. "Red shoes" could be a possible link to Noir, since Kirika, the young girl, also wear red shoes. In episode 25 you see that Madlax is also wearing red shoes. Both pairs of red shoes from both series look exactly alike.
[EASTER EGG]: (U.S. DVD #6) According to ADV's monthly e-newsletter (March 2006), an extra called "Sock Puppet Theater" is included in the English version of volume six. To access it, press up, down, left and right (in that order) when the second eye-catch of episode twenty-one comes on. It's a "live action" about Madlax going after Chris Patton, an annoying womanizer. This extra is in English only.
Yuki Kajiura wrote the score in a high-rise hotel during pre-production which, according to Kajiura, is unusual for her as it is a new environment. Also, according to music producer Keiichi Nozaki it is much cheaper than a sound studio.
Margaret's home country of Nafrece is modeled after France. At several points you can see famous French attractions such as the Eiffel Tower and the Arc de Triomphe. Nafrece is also an inexact anagram of the name France.
Screenwriter Yôsuke Kuroda and director Kôichi Mashimo had worked together before on a different project, however it never made it on the air. It was a year after that he was offered the job to write for this series.
Kôichi Mashimo originally pitched the idea of MADLAX to producer Shigeru Kitayama as a show about a hot female mercenary who would battle armies for treasure. Kitayama brushed it off at first but then purposed revisions to Mashimo's idea. Mashimo asked Kitayama to complete a draft for a first episode and the story was conceived. Shortly after Yôsuke Kuroda was hired to script the series.
Writer Yôsuke Kuroda told in an interview to NewType Magazine that he and director Kôichi Mashimo conceived the ending to the series and the connection between all the characters during an intoxicated brainstorming session.
The name Friday Monday may be a reference to a novel by G. K. Chesterton specifically, to The Man Who Was Thursday, which is about a group of alleged anarchists (in the philosophical rather than political sense) who use the days of the week as code names. Friday Monday and the Council of Days share similar philosophies.
It was producer Shigeru Kitayama's insistence to put a maid character in the series. He also mentioned to director Mashimo that although there are a lot of maids with male masters who the viewers are supposed to empathize with (as in they insert themselves in place of the guy) in anime, he thought that there were probably none with female mistresses.
The trivia item below may give away important plot points.
Yôsuke Kuroda revealed in "Madlax The Bible" that Friday Monday and Charlie do not die in the end but Quanzitta does. Initially there was going to be a scene of Charlie waking up from the trance but the scene was cut.