The decision by the Food and Drug Administration to allow U.S.-manufactured infant formula contaminated with melamine or its byproducts onto store shelves is "seriously flawed" and medically risky because parents may feed their babies more than one product, scientists at the nonprofit group Consumers Union said Friday.

The FDA detected melamine and its byproduct cyanuric acid separately in four of 89 containers of infant formula tested in the fall, but never at the same time. A can of milk-based liquid Nestle Good Start Supreme Infant Formula with Iron contained traces of melamine while three different cans of Mead Johnson's Enfamil LIPIL with Iron had traces of cyanuric acid.

The FDA says studies show potentially dangerous health effects from the industrial chemicals only when both are present. The lack of dual contamination is key, say agency officials, and thus there have been no recalls of the tainted formula.

In a letter Friday, consumer advocates told FDA commissioner Andrew C. von Eschenbach and U.S. Department of Health and Human Services Secretary-Designate Tom Daschle that they were concerned the FDA was assuming parents would never feed their babies more than one type of formula. They said they had heard from a concerned mother who routinely fed her baby two different formulas because "one caused constipation, and one caused loose bowels, but together the baby's digestion seemed just right."

"FDA should regulate infant formula based on an assumption that infants may be exposed to melamine and cyanuric acid in combination," they wrote.