A2 milk is a variety of cows' milk that mostly lacks a form of β-casein proteins called A1, and instead has mostly the A2 form.[1] Cows' milk like this was brought to market by The a2 Milk Company and is sold mostly in Australia, New Zealand, China, the United States, and the United Kingdom.[2] Non-cow milk, including that of humans, sheep, goats, donkeys, yaks, camels, buffalo, and others, also contain mostly A2 β-casein, and so the term "A2 milk" is also used in that context.[3][4]
The a2 Milk Company claims that milk containing A1 proteins is harmful, and some companies producing goat's milk products make similar claims,[5] but a 2009 European Food Safety Authority (EFSA) review found that it could not be established that bioactive peptides in milk containing both the A1 and A2 proteins have an adverse effect on health.[2] - from Wikipedia.
Since being on A2 milk my health has improved, so I swear by it even though it isn't the cheapest.