Wal-Mart Stores Inc. plans to roll out sophisticated electronic ID tags to track individual pairs of jeans and underwear, the first step in a system that advocates say better controls inventory but some critics say raises privacy concerns.

While the tags can be removed from clothing and packages, they can't be turned off, and they are trackable. Some privacy advocates hypothesize that unscrupulous marketers or criminals will be able to drive by consumers' homes and scan their garbage to discover what they have recently bought.

They also worry that retailers will be able to scan customers who carry new types of personal ID cards as they walk through a store, without their knowledge. Several states, including Washington and New York, have begun issuing enhanced driver's licenses that contain radio- frequency tags with unique ID numbers, to make border crossings easier for frequent travelers. Some privacy advocates contend that retailers could theoretically scan people with such licenses as they make purchases, combine the info with their credit card data, and then know the person's identity the next time they stepped into the store.

"There are two things you really don't want to tag, clothing and identity documents, and ironically that's where we are seeing adoption," said Katherine Albrecht, founder of a group called Consumers Against Supermarket Privacy Invasion and Numbering and author of a book called "Spychips" that argues against RFID technology. "The inventory guys may be in the dark about this, but there are a lot of corporate marketers who are interested in tracking people as they walk sales floors.

Wal-Mart is demanding that suppliers add the tags to removable labels or packaging instead of embedding them in clothes, to minimize fears that they could be used to track people's movements. It also is posting signs informing customers about the tags.

Proponents, meanwhile, have high hopes for expanded use in the future. Beyond more-efficient recalls and loss prevention, RFID tags could get rid of checkout lines. "We are going to see contactless checkouts with mobile phones or kiosks, and we will see new ways to interact, such as being able to find out whether other sizes and colors are available while trying something on in a dressing room," said Bill Hardgrave, head of the RFID Research Center at the University of Arkansas, which is funded in part by Wal-Mart. "That is where the magic is going to happen. But that's all years away."