Reasons for the highly variable and often poor protection conferred by the Mycobacterium bovis bacille Calmette-Guerin (BCG) vaccine are multifaceted and poorly understood. The objective of the study was to determine whether early life PCB (polychlorinated biphenyls) and DDE (1,1-dichloro-2,2-bis(p-chlorophenyl)ethylene) exposure reduces 6-month infant BCG vaccine response.