Does Butter Contain Casein?

Yes, butter contains trace amounts of casein. So if one is on a strict casein-free diet, as some children with autism, ADD, and ADHD are, then butter would not be acceptable. Ghee, which is clarified butter, is casein-free. [reference]

The amount of casein that butter contain is not a large amount, however.  Casein is a protein found in dairy.  And if you look at the box label, it typically says 0 grams of protein.

Butter has less casein than milk.  Depending on a person sensitivity, they may be able to tolerate butter even though they may not be able to tolerate milk.