The B vitamins, except for folate, can be classified as group I nutrients during lactation. Nutrients in this category share the following characteristics: low maternal intake or stores during lactation reduce the concentration in human milk, and infants' stores are readily depleted. For some of these nutrients, the infants' stores at birth may be depleted by maternal deficiency during pregnancy. The prevalence of some B vitamin deficiencies, especially deficiencies of riboflavin and vitamin B-12, is probably much higher than is usually assumed. Taken together, these considerations emphasize the importance of supplying adequate amounts of B vitamins to infants and young children. Recommendations are made here on the amounts and densities of B vitamins that should be present in fortified complementary foods fed to children aged 6-24 mo. The values are based on the difference between recommended daily intakes and the amount that the child will receive from maternal milk using estimates reported in the literature. There are few concerns about the potential toxicity of any of these vitamins at the levels likely to be added to complementary foods. If there are losses during food preparation or concentrations of the vitamins are low in human milk, the estimates provided may need to be increased. The adequacy of these recommendations must be evaluated thoroughly.