(I did't really write this, I just got it from wikapedia.)
From 1 May 2007, many Mars products made in the UK became unsuitable for vegetarians. The company announced that it would be using whey made with animal rennet (material from a calf's stomach lining, and a byproduct of veal), instead of using rennet made by microorganisms, in products including Mars, Twix, Snickers, Maltesers, Bounty, Minstrels and Milky Way. The response from many thousands of consumers, particularly the Vegetarian Society's request for UK vegetarians to register their protests with Mars, generated a lot of press, and caused the company to abandon these plans shortly thereafter. It has reportedly decided to switch to all-vegetarian sources in the near future in the UK. In January 2008 the Metro newspaper reported that Mars had allegedly begun to incorporate animal derived rennet.
Mars has come under criticism by People for the Ethical Treatment of Animals for funding animal tests which the group alleges are inhumane, including a study regarding angiogenesis and spatial memory in which mice were force fed catechins, a candy ingredient, compelled to swim in paint, then dissected. Other experiments involved life-threatening cardiac puncture of mice with inadequate anesthesia. These experiments are not required by law. PETA alleges that their only purpose is to promote increased consumption of Mars' products and that harmless tests can be done on humans.