Phosphatidylserine transport in normal and Rhnull red blood cells was determined by measuring characteristic morphologic changes induced by synthetic phospholipids. Treating normal A+ cells with commercial anti- A antisera, anti-Rho(D) antisera, or with saturating concentrations of purified Rho(D) antibodies had no effect on phosphatidylserine transport. Normal B- cells treated with purified anti-B antibodies transported phosphatidylserine at rates equal to those of cells not treated with antibody. Rhnull cells, deficient in the protein bearing the Rho(D) antigen, incorporated dimyristoylphosphatidylcholine and dimyristoylphosphatidylserine at rates and to extents similar to normal cells. Furthermore, incorporated phosphatidylserine, but not phosphatidylcholine, was rapidly transported across the membrane bilayer. Energy depletion or treatment with sulfhydryl reagents inhibited phosphatidylserine transport equally in normal and Rhnull cells. These results indicate that, although Rhnull cells have numerous membrane defects, they are capable of adenosine triphosphate-dependent transport of exogenously added dimyristoylphosphatidylserine. Normal phosphatidylserine transport in the presence of anti-Rho(D) antibodies or in cells deficient in the Rho(D) polypeptide indicates that this protein is not the aminophospholipid transporter.