Patulin is a toxic metabolite produced by various species of Penicillium, Aspergillus and Byssochlamys. In the present study, its effects on the plasma membrane of fission yeast Schizosaccharomyces pombe were investigated. The phase-transition temperature (G) of untreated cells, measured by electron paramagnetic resonance spectrometry proved to be 14.1 degrees C. Treatment of cells for 20 min with 50, 500, or 1000 microM patulin resulted in a decrease of the G value of the plasma membrane to 13.9, 10.1 or 8.7 degrees C, respectively. This change in the transition temperature was accompanied by the loss of compounds absorbing light at 260 nm. Treatment of cells with 50, 500 or 1000 microM patulin for 20 min induced the efflux of 25%, 30.5% or 34%, respectively, of these compounds. Besides its cytotoxic effects an adaptation process was observed. This is the first study to describe the direct interaction of patulin with the plasma membrane, a process which could definitely contribute to the adverse toxic effects induced by patulin.