We followed a group of exclusively breast feeding mothers for 12 months after delivery. They numbered 200 at delivery, 140 at 4 months of lactation, 116 at 6, 36 at 9 and 7 at 12 months. The milk concentration of biotin was very low at delivery and subsequently the (geometric) mean value remained at around 4.5 micrograms/l. The mean plasma value was 250 ng/l at delivery and stable (around 330 ng/l) throughout the lactation. There were large intra- and interindividual variations in the concentrations in both milk (from 0 to 27 micrograms/l) and plasma (from 142 to 1090 ng/l). The variations in the milk concentrations result in a similar variation in the biotin intake of the exclusively breast-fed infants. The current recommendations for biotin intake were not met by any of those infants but no manifestations of biotin deficiency were recorded.