We have previously shown that overexpression of the multidrug resistance (MDR) efflux transporter ABCG2 in the membrane of novel extracellular vesicles that are confined to breast cancer cell-cell attachment zones confers mitoxantrone resistance and mediates a marked intravesicular concentration of an unknown endogenous green fluorescent compound (I. Ifergan, G.L. Scheffer, Y.G. Assaraf, Novel extracellular vesicles mediate an ABCG2-dependent anticancer drug sequestration and resistance, Cancer Res. 65 (2005) 10952-10958). Here we identified the latter as riboflavin (vitamin B2) and further demonstrated that the marked intravesicular concentration of riboflavin in ABCG2-overexpressing breast and lung cancer cells tightly correlates with the extent of ABCG2 overexpression and its differential localization to the vesicular membrane and not to the plasma membrane surrounded by growth medium. We hence propose that the ABCG2-dependent concentration of riboflavin in these intercellular compartments may serve as a novel, sensitive, and non-cytotoxic (i.e. based on vitamin accumulation) functional marker for the quantification of the levels of MDR mediated by ABCG2-rich extracellular vesicles in multiple malignant cells.