Irradiation of plasma membrane preparations with 254 nm light increases its apparent microviscosity with fluorescent polarimetry. Doses of 3 . 10(4) J/m2 increase the fluorescent polarization of a diphenylhexatriene probe by 10%. A similar increase is seen when whole cells are irradiated. The fate of membrane protein following irradiation was examined using SDS-polyacrylamide gel electrophoresis. Increasing the 254 nm doses reduces the amount of material in distinct bands on the gel and increases the amount of very low mobility material. No new bands of Coomassie blue staining material were observed. Irradiation of whole cells inhibited their attachment to concanavalin A-coated surfaces, an indication of a change in membrane function.