Plasma and red cell folate concentrations (lactobacillus casei activity) and other pertinent blood values have been studied during the 1st year of life in 41 premature infants (mean gestational age 31.6, range 26-35 weeks). They were formula-fed, 48.5 nmol (21 micrograms) folate per 1, from 1 month of age. The infants were divided into two groups according to their birth weights (BW): group A, BW less than or equal to 1750 g and group B, BW greater than 1750 g, respectively. One-half of the infants in each group received an extra 113.5 nmol (50 micrograms) folic acid daily. The premature infants were compared with 35 breast-fed term infants considered to have an optimal folate status. The infants not receiving folic acid supplementation had low plasma and red cell folate concentrations during the first months of life, while those receiving supplementation had values comparable to the breast-fed infants. No significant differences in the gain in weight and increase in length were observed when the folic acid supplemented infants in group A were compared with the non-supplemented infants. However, in the case of group B a significant increase in length and a somewhat greater weight gain were observed for infants with folic acid supplementation in comparison with those not given extra folate. No significant differences were observed between the haemoglobin, RBC and VPRC values in the folic acid supplemented and non-supplemented infants. It is estimated that the optimal folate intake during the first months of life in formula-fed premature infants is about 150 nmol (65 micrograms) per day. This amount is higher than previously recommended. The infants from all groups had a folate intake similar to, or above, the minimal daily requirement needed for erythropoiesis.