Question: What is Mittelos?
The term "Mittelos" was first introduced in Episode 3x7, Not in Portland.
Answer: Mittelos Bioscience claims to be a research company. The research company is probably just a front for the work that is being done on the island.
Richard Alpert sought out Juliet to work for Mittelos in her specialty of fertility. Richard said the company was in Portland, but later he said it wasn't quite in Portland.
Juliet couldn't take the job because of her overbearing ex-husband and mentioned the only way she could do it would be if her ex were hit by bus. Later, her ex was hit by a bus, which was most likely arranged by someone at the Mittelos Bioscience company (Ep: 3x7, Not in Portland).
In one of Locke's flashbacks, Richard Alpert claimed to want Locke to go to a science camp put on by Mittelos Laboratories (Ep: 4x11, Cabin Fever).
