Leucocidin, a toxic protein obtained from Pseudomonas aeruginosa, was tested in perfused rat livers. Doses of greater than 4 mug/g liver produced a heavy potassium loss and a massive decrease of the perfusion rate but only moderate swelling of the liver at 37 decrees C. All effects depended on the dose. If the perfusions were carried out at 22 or 9 degrees C neither K+-loss nor swelling, and only a moderate increase of the vascular resistance was observed. Swelling at 37 degrees C was stronger at low initial perfusion rates (2.0 ml.min-1.g-1) than at higher ones (4.0 ml.min-1.g-1). The major effect of leucocidin could be prevented when the perfusion medium was changed 30 min after the addition of the toxin. Only small amounts o of 125I-leucocidin were taken up by the liver tissue. The time course and the quantity of nearly all effects of leucocidin were quite different from those of phalloidin.