The comparative efficacy of antibacterial therapy with pivmecillinam or cotrimoxazole and general supportive care only was studied in patients with severe bacterial gastroenteritis. Overall, treatment with antibiotics proved significantly superior to rehydration alone in 42 children. Active therapy also had a statistically beneficial effect in children infected with Vibrio cholerae and V. parahaemolyticus. Pivmecillinam and co-trimoxazole were equally effective. Pivmecillinam and oral mecillinam appeared to be of equal value in a further 22 adults infected by Vibrio spp. No side-effects were recorded in any of the subjects treated. Further investigations with pivmecillinam and oral mecillinam are advocated.