The volunteer challenge model was used to study the virulence of strains of Vibrio cholerae O1 El Tor recently isolated from cases of cholera in South America. Fifteen of the 24 volunteers (62%) were of blood group O, the group most prevalent in South America and the group at increased risk of more severe cholera. Two El T or Inaba strains and 2 El Tor Ogawa strains were given to volunteers at a dose of 1-2 x 10(6) colony-forming units. All 4 strains caused diarrhoea in 67-83% of volunteers. Volunteers with blood group antigen O had an increased attack rate for diarrhoea (P = 0.015) and a marginally increased mean diarrhoeal stool volume (P = 0.08) after challenge. One-third of the volunteers with blood group O, and none of the volunteers with other blood groups, developed severe diarrhoea (> 5 L) (P = 0.01). This study established a model of South American cholera that can be used to predict field efficacy of candidate vaccines among populations with a high prevalence of blood group antigen O.