Dietary intake, milk composition, growth, and activity were monitored monthly for 20 breast-fed infants from 1 to 6 months. Breast milk intake (of infants receiving no more than 50 kcal from other foods) ranged from 341 to 1,096 ml/day, with mean intake increasing from 673 to 896 ml/day from 1 to 6 months. Energy intake per kilogram body weight averaged 113 kcal/kg/day at 1 month, decreasing to 85-89 kcal/kg/day at 5-6 months--considerably lower than the recommended 115 kcal/kg/day. At 6 months, all infants except one were above the 10th percentile of weight-for-age. Nutrient content of milk samples was similar to previously reported values. Protein, iron, zinc, copper, sodium, and potassium levels declined during the first 6 months. Intakes of iron, zinc, calcium, and magnesium were lower than recommended allowances for infants. Weight-for-length and weight gain were significantly correlated with total energy intake, but not with activity level, during the first 6 months.