A cottage industry has sprung up facilitating the sale and donation of human breast milk on the Internet, but a study published Monday in the journal Pediatrics confirms the concerns of health professionals over this unregulated marketplace.

The report found that breast milk bought from two popular Web sites was often contaminated with high levels of bacteria, including, in a few instances, salmonella. The amounts detected in some samples were sufficient to sicken a child.

“The study makes you worry,” said Dr. Richard A. Polin, the director of neonatology and perinatology at Columbia University, who was not involved in the research. “This is a potential cause of disease. Even with a relative, it’s probably not a good idea to share.”

After a spate of research showing that breast milk protects infants from infections and other ailments, health care providers in recent years have strongly encouraged new mothers to abandon formula and to breast-feed. But this can be a difficult challenge. Parents who have adopted, for instance, or have had mastectomies — or who simply do not produce enough milk — often rely on donated or purchased breast milk.