Early milk samples from 102 American mothers were examined for Ca, P, and Mg contents in relation to stage of lactation, intake of prenatal mineral supplements, maternal age, parity, and previous history of lactation. A total of 415 samples were collected at three stages of lactation: early transitional (4-7 days postpartum); transitional (10-14 days postpartum); and mature (30-45 days postpartum). No diurnal variations in element concentrations were observed in representative samples of late evening (PM) and early morning (AM) feedings collected during the transitional and mature stages. The mean concentrations for the major elements were highest in early transitional milk and in some cases decreased significantly (p less than 0.05) as lactation progressed. Ca, P, and Mg contents (means +/- SEM) were 26.3 +/- 0.6, 14.6 +/- 0.4, 5.3 +/- 0.1 mg/100 g in early transitional milk and 26.2 +/- 0.5, 13.3 +/- 0.3, and 5.0 +/- 0.1 in mature milk, respectively. Increasing uniformity in the elemental content of milk was noted among the mothers as lactation became established. No significant relationship was found between intake of dietary supplements containing Ca and Mg and levels of these elements in milk. Also, no significant correlations were found between maternal age, parity, or previous history of lactation and the elemental content of milk. From these data, it was estimated that fully breast-fed infants would receive approximately 33, 18, and 6.5 mg/kg/day of Ca, P, and Mg, respectively, during the neonatal period.