Samples of maternal blood, milk, and umbilical cord blood were collected from mothers and their infants at Ulleval University Hospital in Oslo, Norway. All the mothers had lived in Oslo during the last two years. Of the Norwegian mothers, 16 were delivered by Caesarean operation, and samples of subcutaneous fat tissues were collected. Hexachlorobenzene (HCB), polychlorinated biphenyls (PCBs),p,p′-DDE,p,p′-DDT, and dieldrin were found in 157, 135, 168, 22, and 12, respectively, of the 168 samples analyzed, while β, and γ-hexachlorocyclohexane (HCH) were found in 93 and 26, respectively, of the 152 samples analyzed for these compounds. In the samples obtained from the Norwegian mothers and infants, PCBs andp,p′-DDE were the major contaminants present. In contrast, only traces of PCBs were found in six immigrant samples, which, however, contained 4–6 times higher levels ofp,p′-DDE plusp,p′-DDT than the corresponding Norwegian samples. Of the HCH isomers, the β-HCH was the predominant isomer present. Two to three times significantly higherp,p′-DDE and PCBs levels were found in maternal serum than cord serum. No significant differences were found between the organochlorine residue levels in subcutaneous fat and milk fat. A significant positive linear correlation was found between the levels ofp,p′-DDE and PCBs in maternal serum and in milk fat, and between the levels ofp,p′-DDE in maternal serum and cord serum from Norwegian mothers giving birth normally. Significantly higher PCBs andp,p′-DDE residues were found in the maternal and cord serum samples obtained after Caesarean operation than after normal delivery, while no other such differences were found in the milk samples 5 days post partum. The results demonstrate that organochlorines are transferred from mother to fetus and newborn babies through the placenta and milk and that human milk is a good indicator in monitoring human beings and their environment for organochlorine contamination. Possible toxicological implications of the present results are discussed.