Two hundred sixty-seven samples of breast milk, supplied by 140 donors from both urban (N = 45) and rural (N = 95) areas, were analyzed on a whole milk basis for aldrin, gamma-benzene hexachloride (gamma BHC), hexachlorobenzene (HCB), dieldrin, and total dichlorodiphenyltrichloroethane (DDT) to ascertain the effectiveness of restrictions placed on pesticide use since 1971. Aldrin and gamma BHC were found to be present mainly in trace amounts, while decreases were found with HCB (0.025-0.008 ppm) and total DDT (0.078-0.046 ppm). Dieldrin showed an increase (0.005-0.009 ppm) which caused some concern. The results of a questionnaire suggested a connection between the high dieldrin levels and a program of treatment for the protection of houses against termites. A statistical analysis further supported this theory and suggested three variables being significantly associated with dieldrin levels: (1) treatment of the house, (2) suburb in which the donor lived, and (3) age of the mother. Further relationships were shown to exist between pesticide levels, lipid content of the milk, and the stage of feeding at which the sample was taken.