Lead levels in breast milk and blood were determined in women living within a 200-m radius of 3 smelters in Mexico City. All samples were analyzed on a Perkin Elmer 460 atomic absorption spectrometer equipped with HGA 2200. The mean blood lead level was 45.88 micrograms/dl (SD 19.88 microgram/dl), and the geometric mean of milk lead level was 2.47 micrograms/100 ml. The correlation coefficient of these two variables was 0.88. Using the mean value of lead found in breast milk, an infant of 5.5 kg would ingest 8.1 micrograms/kg/d in his diet. The daily permissible intake (DPI) established by the World Health Organization (WHO) in 1972 for an adult is 5.0 micrograms/kg/d.