The FDA ruling has been in the works for at least four years, and during that time the agency has asked cattle and dairy farms with cloned animals to keep them out of the food supply. Farmers and ranchers have complied, in part because these animals are too expensive to turn into hamburger; it costs about $20,000 to clone a cow. So observers expect that if anything ends up on supermarket shelves, it won't come directly from clones but from their offspring, who come into the world in a more or less natural way. Clones would be used to start lines of superior meat- and milk-producing animals. (Some clones could become burgers, though, after they've become old and can't breed anymore. Animals past their breeding prime are often candidates for the slaughterhouse.)

Those genetic lines are another worry for Mendelson. "If you create more genetic uniformity in a herd, you're also creating more susceptibility to diseases," he says. "We know that from history. So then you'll need more antibiotics and more hormones to keep the herd healthy, and those may carry their own risks."

Wheeler doesn't agree. "You won't get any more genetic uniformity than you do with current breeding techniques," he says, noting that perhaps 70 percent of today's dairy cows are produced using in vitro fertilization or other artificial methods. "Plus, we screen cows now for traces of antibiotics, and if we find them we don't use the animals. So there's already a system for keeping them out of the food supply." He adds that market forces will keep farmers from raising sickly animals who need antibiotics and hormones: The cost of these substances will eat into their profits. It's much cheaper to raise healthy animals.

As for marking meat "cloned" or "clone-free," the FDA's Sundlof says the agency has made no decision yet. "But if our risk assessment isn't altered, it would be unlikely for us to require any special labeling," he says.