The relationship of natural and artificial feeding to serum and red cell folate levels, as a means of evaluating nutritional intake of this vitamin, was studied. First a cross-sectional study was undertaken in which a group of 112 infants still being breast-fed at 4, 6, and 9 months of age had significantly higher folate concentrations than another group of 102 infants who had been weaned before 2 months. At 6 months of age the prevalence of folic acid deficiency (red cell folate less than 160 micrograms/l of erythrocytes) was 0% in the first group and 15.4% in the second (P less than 0.02), and at 9 months the respective proportions were 3.6 and 28.9% (P less than 0.01). In a second study, 50 nursing infants were followed longitudinally during their first 15 months of life, and up to 9 months of age a direct correlation was observed between the folate concentrations and the duration of breast-feeding. In order to prevent a deficiency of this vitamin, it is recommended that infants be exclusively breast-fed at least through the age of 6 months or, failing this, that they be fed infant formula enriched with folic acid.