Photodynamic treatment of yeast with the sensitizer Toluidine blue results in loss of cell viability. In previous investigations it was suggested that plasma membrane damage might be responsible for the loss of colony forming capacity. In this context the influence of photodynamic treatment on transmembrane transport systems was studied. It appeared that the uptake of the sugars glucose, lactose and galactose, the amino acids arginine, phenylalanine, glycine and aspartic acid and of the inorganic compound phosphate was inhibited by photodynamic treatment. The different elements of the energy providing system necessary for active transport, viz. the plasma membrane ATPase and the protonmotive force, were not significantly affected by Toluidine blue and light, indicating that inhibition of transport is not caused by a reduction of the membrane potential or the transmembrane pH gradient. These observations suggest that the transport carriers themselves were damaged by treatment with Toluidine blue and light. This could be confirmed in experiments, in which the lactose and galactose transport proteins of treated and untreated cells were reconstituted in plasma membrane vesicles. It appeared that the carriers, obtained from photodynamically treated Kluyveromyces marxianus cell, had lost their transport capacity.