Hexachlorobenzene (HCB), a ubiquitous lipophilic pollutant, was found to be readily transferred in the milk of lactating dams to their suckling neonates. Pregnant CD-1 mice were treated during gestation, and the body burdens of HCB in the neonates and the dams were determined during lactation. Also, neonates from dams treated with HCB during gestation were cross-fostered at birth to dams treated with corn oil during gestation. The body burdens of HCB were greater in the neonates exposed to HCB by lactational transfer than the neonates exposed only by gestational transfer. For a number of tissues, the body burden of HCB in the pups was similar for full litters and litters reduced to 2 pups per litter. The body burdens of the dams were greatly reduced during lactation with a loss of 95% of HCB. A similar reduction was seen in dams with litters reduced to 2 pups. Lactational transfer of HCB was a major route of excretion. Non-pregnant mice were studied concurrently.