The penetration into the stratum corneum of fluorescein, as the acid form or as a sodium salt, encapsulated in liposomes formed by liquid- or gel-state phospholipids, with or without cholesterol, was investigated in humans by the stripping method. Liposomes prepared by extrusion were applied to the forearms of healthy human volunteers and 30 min later, strippings were performed. Fluorescein was extracted and determined by spectrofluorimetry. The skin penetration of sodium fluorescein was higher from fluid liposomes (phosphatidylcholine) than from rigid liposomes (hydrogenated phosphatidylcholine), but it was independent of the content of cholesterol. It seems that the liquid-crystalline state of the lipids is the main aspect involved in the fluidity of the liposome bilayer itself as well as in the interaction with the lipids of the stratum corneum. The similar enhanced penetration behavior obtained for unsaturated liposomes containing sodium or acid fluorescein seems to support the hypothesis of a previous destruction of the vesicles during its passage through the lipid intercellular pathway in the stratum corneum.