Previous studies have shown that the classical isolates of Vibrio cholerae possess an 11 bp deletion in the structural gene for the El Tor haemolysin leading to the production of a 27 kDa non-haemolytic truncated product HlyA* compared to the 82 kDa haemolysin, HlyA. These studies were designed to assess whether this truncated product had any biological activity. A KmR cartridge was introduced into the hlyA gene effectively eliminating the haemolysin. This was recombined into the chromosome of a variety of strains and isogenic pairs were examined in a number of systems. These studies suggest that the haemolytic (cytolytic) domain of HlyA resides at the C-terminus and that the N-terminus, which is conserved as HlyA* in classical strains, possesses enterotoxic (cytotoxic) activity. Experiments with the cholera-toxinless vaccine candidate JBK70 and its hlyA::KmR mutant suggest that HlyA* may be responsible for the residual diarrhoea observed in cholera-toxinless vaccine strains.