As chemical companies develop more pesticides, cleaners, and other potentially toxic compounds, traditional methods of safety testing can hardly keep up. Animal tests, which have been the gold standard for decades, are slow and expensive, and these sorts of tests are increasingly socially unacceptable, too. What's more, the results of animal testing sometimes don't translate to humans, so researchers are eager for better alternatives.
http://tech1207.blogspot.com/2008/02/improving-toxicity-tests.html