Breast milk samples were collected from mothers living in areas having an average of 0.2 and 1.7 microgram fluorine/ml in drinking water. The samples were analysed for their ionic and total F concentrations. 2. Ionic F was determined after microdiffusion by the fluoride-selective electrode. For total F determinations, milk samples were ashed with magnesium succinate fixative, after which the F concentration was determined as mentioned previously. 3. The median totals for F content of breast milk in the low- and high-F areas were 7.0 ng/ml (range 4.3-14.0 ng/ml, fifteen mothers) and 10.9 ng/ml (range 4.5-50.7 ng/ml, nine mothers). The median ionic F concentrations were 5.0 ng/ml (range 3.0-6.3 ng/ml, fifteen mothers) and 8.9 ng/ml (range less than 2.0-39.9 ng/ml, nine mothers) respectively. Both the total F (0.01 greater than P greater than 0.001) and ionic F (0.05 greater than P greater than 0.01) in breast milk were significantly higher in the high-F area compared to the low-F area. 4. At the ages of 1,2 and 3 months, the calculated average total daily F intakes of exclusively breast-fed infants were 4.8 and 7.3 microgram; 5.0 and 7.8 micrograms; 5.4 and 8.5 micrograms in the low- and high-F areas respectively.
