To determine whether perinatal exposure to "background" levels of polychlorinated biphenyls (PCBs) or dichlorodiphenyldichloroethene (DDE) affected child development, we followed a cohort of children through the age of 2 years; 676 were tested at 18 months and 670 at 24 months with the Bayley Scales. Transplacental and cumulative exposures from breast milk to both chemicals were estimated for each child from analyses of breast milk and other samples. Bayley testing was done without knowing the chemical levels. At 18 and 24 months, adjusted scores on the psychomotor scales were 4 to 9 points lower among children in the top fifth percentile of transplacental PCB exposure, significantly so at 24 months. There were no consistent effects apparent from exposure to PCBs through breast milk, nor from DDE exposure. We conclude that there is a small delay in motor maturation attributable to transplacental exposure to PCBs that is still detectable at 24 months. There is no evidence of an effect from the larger but later exposure through breast milk, although effects may emerge as the children age.