The effects of vitamin supplements and/or diet on the levels of vitamin C, vitamin B6, and vitamin B12 in milk and blood of lactating women were determined. At the end of gestation, subjects were divided into two lactation groups: supplemented (10 subjects) and nonsupplemented (seven subjects). Milk samples were collected from 5 to 7 days and 43 to 45 days postpartum. Fasting blood samples were drawn at 8 and 46 days postpartum for vitamin C, B6, and B12 status measurements. Dietary records of all foods consumed by the subject were kept for 4 days at 1 and 6 weeks postpartum. The vitamin B6 level in breast milk of the unsupplemented group of mothers was significantly lower (P less than 0.05) than the supplemented group of women at 5 to 7 days postpartum. Vitamin B12 concentration in milk of nonsupplemented mothers at 43 to 45 days postpartum was significantly lower (P less than 0.05) than the supplemented group of women at 43 to 45 days postpartum. None of the milk values or the maternal blood levels measured in the women was less than published norms for vitamin C, vitamin B6, and vitamin B12.