The Shiga toxin B-subunit has been fused to the 23-kD C-terminus of Escherichia coli haemolysin A (HlyA) and exported from attenuated antigen carrier strain of Salmonella typhimurium aroA (SL3261). The expression of the gene fusion under the control of a synthetic modified beta-lactamase promoter (constitutive expression) and under the iron-regulated aerobactin promoter showed that the fusion protein could be stably expressed and exported out of the bacterial cell in significant amounts so long as high copy number plasmids were not used. Oral and i.p. immunization of mice with the hybrid salmonellae resulted in significant B-subunit specific mucosal and serum antibody responses. A comparative analysis of the location of hybrid proteins in the antigen carrier bacterial cell (i.e. cytoplasmic expression and extracellular export) has shown that both modes of expression result in antigen-specific immune responses. This is the first report demonstrating that foreign polypeptides fused to the 23-kD C-terminus of E. coli haemolysin A can be exported from attenuated Salmonella vaccine strains and that such exported polypeptides can result in antigen-specific immune responses.