Ro09-0198 is a cyclic peptide isolated from Streptoverticillium griseoverticillatum which recognizes strictly the structure of phosphatidylethanolamine (PE) and forms an equimolar complex with the phospholipid on biological membranes. To use the peptide as a probe for analyzing the transbilayer movement of PE, we labeled the amino-terminal amino acid of the peptide with biotin without changing either the reactivity or specificity of the peptide. The amount of the peptide bound to the membrane was measured by enzyme linked immunosorbent assay (ELISA) after extraction of the peptide from the membrane. The peptide showed a strict temperature-dependent binding to human erythrocytes and the binding increased with increasing temperature. Since the peptide bound to PE in a temperature-independent manner and the binding to membrane PE is not affected by membrane proteins, the present temperature-dependent binding of the peptide to the cell membranes was likely to reflect temperature-dependent translocation of PE. The binding of the peptide to erythrocytes differed greatly among animal species. The peptide also showed temperature-dependent binding to a human histocytic lymphoma cell line, U 937, suggesting that the peptide will provide a novel and convenient probe for analyzing the transbilayer movement of PE in eukaryotic cells.