Thirty schoolgirls, aged 6 to 12, with recurrent Escherichia coil bacteriuria were treated with a new β-lactam antibiotic of the amidino-penicillanic acid series, pivmedilhinam (FL 1039) in a dose of 400 mg 3 times daily for 1 week. The infecting organism was sensitive to the drug in 23 (77%) of the children. The cure rate 3 months after treatment was 63% whereas 70% of the children infected with sensitive strains were cured. The comparable cure rates following previous 7-day courses of Cotrimoxazole (2 tablets twice daily) and ampicillin (250 mg 4 times daily) given to the same group of girls were 53 and 40%, respectively. Although the differences between these cure rates were not significant, the data establish that pivmecillinam is a safe and potent drug in the treatment of urinary tract infection.