A study conducted by Spanish and Moroccan scientists that analyzed 20 types of cow's, goat's and sheep's milk, published in the May 2011 Journal of Agricultural and Food Chemistry has found that a single glass of milk contains residues of up to 20 different chemicals including steroids as well as antibiotics and growth hormones.

http://www.huffingtonpost.com/2011/07/07/milk-painkillers-antibiotics-hormones_n_892257.html
http://pubs.acs.org/page/jafcau/about.html