ABCG2/BCRP is a member of the adenosine triphosphate–binding cassette (ABC) transporter family and is expressed in intestine, kidney, and liver, where it modulates the absorption and excretion of xenobiotic compounds. ABCG2 is also expressed in hematopoietic stem cells and erythroid cells; however, little is known regarding its role in hematopoiesis. Abcg2 null mice have increased levels of protoporphyrin IX (PPIX) in erythroid cells, yet the mechanism for this remains uncertain. We have found that Abcg2 mRNA expression was up-regulated in differentiating erythroid cells, coinciding with increased expression of other erythroid-specific genes. This expression pattern was associated with significant amounts of ABCG2 protein on the membrane of mature peripheral blood erythrocytes. Erythroid cells engineered to express ABCG2 had significantly lower intracellular levels of PPIX, suggesting the modulation of PPIX level by ABCG2. This modulating activity was abrogated by treatment with a specific ABCG2 inhibitor, Ko143, implying that PPIX may be a direct substrate for the transporter. Taken together, our results demonstrate that ABCG2 plays a role in regulating PPIX levels during erythroid differentiation and suggest a potential role for ABCG2 as a genetic determinant in erythropoietic protoporphyria.