9 children presenting with an ampicillin-sensitive coliform urinary tract infection were treated with talampicillin using 1 week of full-dose treatment followed by low-dose prophylaxis. The bowel coliforms were ampicillin-resistant at the start in one girl and became resistant in the remaining 8 within 4 months. During a total of 44 months of talampicillin therapy, 6 girls (2 with vesico-ureteric reflux) developed a symptomatic re-infection of the urinary tract, a recurrence rate of 1 per 7.3 months, or 1.6 recurrences per annum. A further 12 girls were given prophylactic talampicillin, 9 after an initial therapeutic course of co-trimoxazole for 1 week and 3 following a period of prophylaxis with low-dose co-trimoxazole. The rectal swab from one girl showed partial ampicillin resistance but 9 of the remaining 11 showed that a predominance of ampicillin-resistance coliforms had emerged in the bowel flora within 4 months. 5 of the 12 also developed a symptomatic ampicillin-resistant urinary infection within 4 months, a recurrence rate of 1 per 7.1 months or 1.7 recurrences per annum. Talampicillin, though very effective in treating urinary infection, is not recommended for the prevention of subsequent recurrence.