This book is based on experimental work performed at Aarhus University. The work was conducted to gain insight into the cellular endocytosis and export of vitamin B12. Vitamin B12 (cobalamin) is an organic molecule which functions as an essential coenzyme for two metabolic reactions in mammalians. Cellular vitamin B12 uptake is a complex process involving different binding proteins and receptors. The studies revealed that the endocytosis of transcobalamin bound to fluorescent vitamin B12 conjugate is correlated with cell growth in fast dividing cancer cells and the uptake of free vitamin B12 was negliable at low as well as high cell division activity. Furthermore this study provides evidenced that vitamin B12 is exported to the cellular environment as a free molecule and the binding to a carrier protein occur subsequently to export, and a novel transporter for the cellular export of vitamin B12 was identified. In conclusion, this PhD study provides new information on the cellular vitamin B12 import and in particular export by revealed a novel transport mechanism of non-protein bound vitamin B12 and the identification of a transporter protein for this process.