Antimicrobial peptides found and isolated from a wide variety of organisms have an activity to kill bacteria. The target of these peptides is thought to be the lipid membrane region of the bacterial and fungal biomembranes. Using the single GUV (giant unilamellar vesicle) method, we have succeeded in revealing the elementary processes of the pore formation in lipid membranes induced by antimicrobial peptide, magainin 2. The statistical analysis of the pore formation in a GUV over many ?single GUVs? enabled us to estimate the rate constant of the magainin 2-induced pore formation in lipid membranes. In this report, to reveal the size of the magainin 2-induced pores in lipid membranes, we investigated the interactions of magainin 2 with single GUVs containing various sizes of fluorescent probes. Under the conditions with no photobleaching of fluorescent probes, we investigated the interaction of magainin 2 with single GUVs of 50% dioleoylphosphatidylglycerol (DOPG)/ 50% dioleoylphosphatidylcholine (DOPC) membrane containing various sizes of fluorescent probes in 10 mM PIPES (pH 7.0), 150 mM NaCl (buffer A). Magainin 2 induced a transient (less than 10 s), but very small (10-20%) leakage of Texas-Red dextran 40,000 (TRD-40 k), Texas-Red dextran 70,000 (TRD-70 k), and FITC-BSA, although the same concentrations of magainin 2 induced a complete leakage of calcein. In contrast, the magainin 2-induced leakage of Texas-Red dextran 10,000 (TRD-10 k) and Texas-Red dextran 3,000 (TRD-3 k) had two phases; the transient rapid leakage in the initial stage and the following slow leakage. These results indicate that in the initial stage of the magainin 2-induced pore formation, the size of the pore was large and then transformed into a small, steady size. The radius of the transient, large pore in the initial stage is larger than 6.4 nm (i.e., the Stokes-Einstein radius, RSE, of TRD-70 k) and also that the radius of the small steady pore in the final stage is smaller than 3.5 nm (i.e., RSE of FITC-BSA). The amount of the leakage of TRD-10 k in the initial stage increased with an increase in magainin 2 concentration. This result indicates that the radius of the large pore in the initial stage increased with an increase in magainin 2 concentration. We discuss these results from a point of view of the pore formation.