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The light-mediated effects of Rose Bengal and 3,3',4',5-tetrachlorosalicylanilide (T4CS) on porcine erythrocyte membranes have been investigated. Irradiation in the presence of Rose Bengal and oxygen causes extensive destruction of the unsaturated fatty acids of the erythrocyte membrane. This results in a decrease in the membrane flexibility as measured by a nitroxide spin probe. Irradiation in the presence of T4CS and oxygen had no measurable effect on the unsaturated fatty acids or on the membrane flexibility. Irradiation of erythrocyte membranes both in the presence of Rose Bengal and oxygen and of T4CS gave rise to polymerisation of the membrane proteins. This was evident by polyacrylamide gel electrophoresis and by freeze-fracture electron microscopy. Aggregation of membrane proteins could be observed with low levels of Rose Bengal and short irradiation times at which no loss of unsaturated fatty acids could be detected. Irradiation of the n-butanol-extracted apoprotein of porcine erythrocyte membranes both in the presence of Rose Bengal and of T4CS resulted in polymerisation of the protein as measured by gel electrophoresis and electron microscopy. The results obtained from the two photodynamic compounds are compared in terms of their different mechanisms of action on the membrane. The implications of the results in determining the molecular events leading to photohaemolysis are discussed.