The trial is a randomized, prospective double-blind trial in which breastfed infants will receive 200 IU/day or 400 IU/day or 600 IU/day or 800 IU/day from 1 to 9 months of age. There will not be a placebo control group. Infants will be followed through 12 months of age. Normal term infants (birth weight >2500 g) who are born in June through November will be enrolled and randomized at 1 month of age. They will visit the study center at monthly intervals and will have capillary blood drawn at select ages. At 2 months and again in March or April whole body mineral content will be determined by DEXA. Infants will not be permitted to receive formula until 9 months of age. They may receive complementary foods beginning at 4 months of age. Vitamin D supplements will be weighed before dispensing and again when the empty and half-empty containers are returned. Study endpoints will be blood parameters and bone mineral content determined at the end of winter, i.e., between March and May 15. Blood parameters include 25-OHD, parathyroid hormone, calcium, alkaline phosphatase, osteocalcin, N-telopeptide, ferritin and transferrin receptor. 180 infants will be enrolled at 1 month of age in the expectation that 48 per group will complete the trial to at least 9 months of age.