Human milk samples obtained from 1436 women residing in the United States were analyzed by gas-liquid chromatography for the chlorinated hydrocarbon insecticides--dieldrin, chlordane, heptachlor, heptachlor epoxide, oxychlordane and Mirex. Dieldrin was found above the detection limit (1.0 ppb) in over 80% of all the samples collected. Chlordane was not found in any of the 1436 milk samples but its metabolite, oxychlordane, was found above the detection limit in 74% of the samples. Although heptachlor was recovered in less than 2% of the samples, its metabolite, heptachlor epoxide, was found above the detection limit in 63% of the samples. It was of considerable interest that Mirex was not positively identified in any of the samples. The proportion of samples with values above the detection limit for dieldrin, heptachlor epoxide and oxychlordane varied significantly among the five geographic regions with the southeastern United States having the highest mean residue level for each of these three contaminants. For the United States the mean fat adjusted residue levels of those samples above the detection limit were 164.2 ppb for dieldrin, 91.4 ppb for heptachlor epoxide, and 95.8 ppb for oxychlordane.