In order to further characterize membrane alterations in human erythrocytes subjected to photodynamic treatment the passive transbilayer mobility of a phospholipid analogue was studied in cells illuminated for various lengths of time in the presence of the photosensitizer, aluminum chlorotetrasulfophthalocyanine. These measurements were combined with the characterization of the membrane leaks for polar solutes occurring under the same conditions with respect to their apparent size, number and ion selectivity. The time-dependent photodynamic enhancement of leaks for K+ as well as choline or erythritol was paralleled by a marked increase of the transbilayer reorientation rate of the amphiphilic lipid probe, palmitoyllysophosphatidylcholine from 0.05% min-1 in native cells to 0.32% min-1 after 60 min illumination. The asymmetric orientation of native phospholipids was not affected by this treatment. The leak permeability proved to be due to the formation of pores with apparent radii of about 0.45 nm after 60 min illumination, and of 0.75 nm after 90 min. The number of pores per cell was calculated to be less than 1, the pores are slightly cation-selective (PK/PCl approximately 3:1). Since photodynamic treatment did not induce lipid peroxidation under the prevailing experimental conditions, protein modification must be the primary cause of both, leak permeability and flip enhancement. Since it is also likely that the leak permeability arises from oxidation of intrinsic membrane proteins, the results raise the interesting possibility that oxidative alteration of intrinsic membrane proteins may lead to enhanced transbilayer mobility of lipids.