The presence of environmental contaminant chemicals in human milk, their demonstrated toxicity, and the lack of data in human beings led to the North Carolina Breast Milk and Formula project, a three-center prospective birth cohort study of 856 children. In this study, we measure polychlorinated biphenyls (PCBs) and DDE in milk and other fluids, follow the course of lactation, and note growth, morbidity and development in the children. Lactation is hormonally complicated and has parts that are plausibly interfered with by contaminant chemicals, and certain kinds of morbidity that occur in breastfed children might also represent the result of chemical contamination of milk. Preliminary data analysis confirms the widespread presence of chemicals in milk; women with higher DDE levels do not breast-feed as long, but this is not true for women with higher PCBs. Besides this study, several case-control or survey-type studies, such as studies of failure to thrive, certain rashes, or short-term breast-feeding would be helpful. Laboratory studies of enzyme induction are now feasible in children and might be a very sensitive if not totally specific endpoint for study of PCBs in milk.