Increased plasma membrane permeability induced by a purified cytotoxic protein from Pseudomonas aeruginosa was studied using mouse Ehrlich ascites tumor cells in incubation medium containing an osmotic stabilizer. In the presence of serum albumin, 40 nM of the cytotoxin was required for cationic imbalance of 2.7 X 10(7) cells per ml at pH 7.4. The rate of passive flux of water-soluble markers with a molecular radius range between 0.3 and 4 nm was used to calculate the size of functional transmembrane pores. Within a short time of intoxication pores of 1 nm in radius were formed. Their stability is inferred from the constant rate of leakage of the most restricted marker during intoxication (60-90 min). The cytotoxin-induced plasma membrane damage led to loss of essential low molecular weight substances and was associated with a decrease of tumor propagation rate in mice. Regression analysis of these functional parameters indicate the reversibility of plasma membrane disorganization up to complete breakdown of the Na+/K+ gradient. The cell can even tolerate partial loss of larger cytosolic compounds under conditions of limited intoxication.