For more than ten years, pet owners have been implanting similar devices in their animals to help locate them when they are lost.

The microchips, generally about the size of a grain of rice, are based on "passive RFID technology," and can be detected with a special device. When a pet shows up at an animal shelter, a technician passes a wand over the pet. If a chip is implanted, a code unique to that pet is displayed -- and in most cases, the animal's owner can easily be found.

But a study by the Dow Chemical Company in 1996 found malignant tumors in a small but significant percentage of lab animals, according to the AP. Another study in 2001 by the Bayer Corporation and published in a journal of toxicology found tumors surrounded the microchips in about one percent of lab rats.

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