Vibrio parahaemolyticus is an important enteric pathogen that produces an exotoxin prepared as Kanagawa haemolysin (KH). Isotope flux techniques were used to analyse toxin action on the basal permeability of human erythrocytes. KH induced a cation leak that was (i) rapid in onset (lag phase < 1 min), (ii) 'pore-like' in terms of kinetic characteristics, and (iii) of high magnitude initially (first 10 min) and then subsequently lower (but still raised with reference to control cells). The susceptibilities of the induced flux pathway to washout in initial and later periods suggested a protracted binding time course for toxin action. Neuraminidase treatment of erythrocytes enhanced both haemolysis and flux induced by KH, suggesting that the affinity of the toxin for the membrane had increased, possibly as a result of additional toxin receptors being unmasked by this enzyme. These results show that KH elevates the basal permeability of human erythrocytes in a complex manner, a process that probably underlies the deleterious effects of this toxin on cellular function.