The purpose of the present study was to monitor the vitamin status of 14 low-birth-weight (LBW) infants (< 1,750 g birth weight) at 2 weeks and an additional four infants at 3 weeks who were receiving an enteral formula providing 247 micrograms/100 kcal thiamine, 617 micrograms/100 kcal riboflavin, 37 micrograms/100 kcal folate, and 0.55 micrograms/100 kcal vitamin B12. The mean birth weight of the 18 infants was 1,100 +/- 259 g, and mean gestational age was 29 +/- 2 weeks. Weekly blood, 24-h urine collections, and dietary intake data were obtained. For thiamine, red blood cell (RBC) transketolase activity was within the normal range for all infants. For riboflavin, RBC glutathione reductase activity was normal for all infants except one. We calculated from intake and urinary excretion data that these infants require 225 micrograms/100 kcal thiamine and 370 micrograms/100 kcal riboflavin, respectively. Mean plasma folate levels were 21 +/- 11 ng/ml at 2 weeks and 18 +/- 5 ng/ml at 3 weeks. RBC folate levels were 455 +/- 280 ng/ml at 2 weeks and 391 +/- 168 ng/ml at 3 weeks. All folate blood values were normal, except for one subject with an elevated level (59 ng/ml). Vitamin B12 plasma values were 737 +/- 394 pg/ml at 2 weeks and 768 +/- 350 pg/ml at 3 weeks, and all values were normal except for three infants with elevated values. In conclusion, appropriate vitamin status was maintained during this short observational period, during administration of this enteral formula; however, riboflavin concentrations in the enteral feed may be excessive.