Fifty-seven patients, twenty-six males and thirty-one females, aged 6 to 50 years (mean 12.6 years) with proven Salmonella typhi or S. paratyphi A septicaemia, were treated in an open randomized parallel study with either aztreonam or chloramphenicol. Aztreonam was given intramuscularly at a level of 50 to 80 mg/kg body weight per dose every 8 h for 7 days to thirty patients. Chloramphenicol was given orally in a dose of 50 to 70 mg/kg body weight every 6 h for 12 days to twenty-seven patients. All patients responded rapidly to treatment, becoming afebrile and asymptomatic within 5.5 to 6.4 days. Only one patient on aztreonam relapsed following treatment, whereas three patients relapsed after chloramphenicol treatment. There were no serious side effects with either drug.