BACKGROUND: There is limited research to determine whether vitamin B12 (B12) supplementation during pregnancy and lactation is protective against oxidative stress and pro-inflammatory cytokines and whether this effect is transferred to breastfed infants via milk. In addition, associations among maternal plasma/ milk and infant B12 status and immune function markers are poorly characterized. OBJECTIVES: To evaluate effects of oral B12 supplementation during pregnancy and postpartum on maternal and infant 8-hydroxy-2-deoxyguanosine (8-OH-dG, an oxidative stress marker) and proinflammatory cytokine levels, and examine associations between maternal plasma, breastmilk and infant B12 status as well as immune function markers. METHOD: In a blinded, placebo-controlled trial, Bangladeshi women (n = 68, 18-35 years, hemoglobin < 11 g/dL, gestational weeks 11-14) received either 250 μg/day B12 or placebo throughout pregnancy up to 3-months postpartum. Samples were collected from mothers at baseline and 3-months postpartum and from infants at 3-months to measure B12 status indicators, 8-OH-dG and proinflammatory cytokines. RESULTS: Maternal postpartum B12 was positively associated with infant plasma B12. Higher milk B12 concentrations were associated with increased infant B12 (beta (β) = 277, 95% confidence interval (CI) = (132, 423), p<0.001) and lower total homocysteine (β = -7.63, 95% CI = (-12.40, -2.86), p = 0.002) levels. Maternal B12 supplementation reduced plasma 8-OH-dG concentrations among postpartum mothers and infants compared to the placebo group. Supplementation increased plasma TNF-α and IL-6 levels among mothers and IL-10 and IFN-γ levels among infants. CONCLUSION: Milk and maternal plasma B12 at 3 months were associated with infant B12. Maternal B12 supplementation modulates 8-OH-dG and several cytokines which may protect against immune response-induced oxidative stress. TRIAL REGISTRATION: (clinicaltrials.gov: NCT01795131- 1st posted on 20/02/2013).