Four major retailers — GNC, Target, Walmart and Walgreens — have been asked to stop selling certain store-brand herbal supplements after tests failed to detect plant materials listed on the label.

New York Attorney General Eric T. Schneiderman said on Tuesday that his office had sent letters to the four retailers, advising them that DNA testing could not verify that the products contained the labeled substances, or found that they contained ingredients not listed on the labels.

The supplements covered by the cease and desist letters include Echinacea, Ginseng, St. John's Wort and others.

Attorney General Schneiderman asked the companies to provide detailed information relating to the production, processing, testing and quality control of herbal supplements sold at their stores.

According to the letters, just 21 percent of the test results from store-brand herbal supplements verified DNA from the plant species listed on the products' labels, while 79 percent did not find DNA related to the labeled content or verified contamination with other plant material.

Many of the tests did not reveal any DNA from a botanical substance of any kind. In some supplements, unlisted contaminants were the only plant material found in the product samples. These contaminants included rice, beans, pine, citrus, asparagus, primrose, wheat, houseplant and wild carrot.

The tests were performed as part of an ongoing investigation by the Attorney General's Office.

"This investigation makes one thing abundantly clear: the old adage 'buyer beware' may be especially true for consumers of herbal supplements," Attorney General Schneiderman commented.