A majority of people who think this is wrong concentrate, sometimes subconsciously, on their need to think that humans are superiorand that human parts can't or shouldn't be grown in animals. Very few think that the unethical part has to do with the inhumane testing on animals. Fewer argue about playing god, which can also be tied into the first arguement more often than not. I think that this testing, if done in humane ways that didn't hurt the animals, would be perfectly ethical and acceptable. It helps to illustrate the shrinking gap between the human and animal world and makes some peope realize that we aren't so diffrent to begin with.