Vitamin B12 is important for infant growth and supplementation to lactating mothers could increase intakes to exclusively breastfed (EBF) infants in areas where maternal diet is poor in B12. HIV+ mothers in the Breastfeeding, Antiretrovirals, and Nutrition study were randomized to receive a lipid‐based nutrient supplement (LNS), containing the RDA for B12 (2.6μg), or no LNS. We examined the relationships among maternal and infant plasma (MP, IP) and breastmilk (BM) concentrations of B12 in a sub‐sample (n=521) at 2 or 6 wk (baseline) and 24 wk. At baseline MP B12 was positively correlated with BM (r=0.36) and IP (r=0.42), and IP and BM B12 were correlated (r=0.32) (all p<0.001). At 24 wk the relationships remained significant: MP B12 with BM (r=0.42) and IP (r=0.32), and IP with BM (r=0.29). At baseline there were no differences between LNS and no LNS in MP, BM or IP B12 concentrations. At 24 wk MP and BM B12 were higher in the LNS vs. no LNS group (p<0.005).However IP B12 was not increased by LNS despite strong correlation among IP, BM and MP. IP B12 is likely explained by maternal status in pregnancy. Maternal supplementation with 1 RDA of B12 was insufficient to improve IP concentrations through breast milk in this population, suggesting that higher levels of supplementation may be necessary.