Cefaclor or amoxicillin was given to 51 randomly assigned patients with urinary tract infections. Inclusion in the study required two pretreatment urine cultures yielding the same organism (susceptible to both antibiotics) in concentrations greater than or equal to 10(5)/ml. Both drugs were administered as 250 mg. orally every eight hours for ten days. Cefaclor was given to 27 patients and amoxicillin to 24. Most patients in both groups had negative urine cultures at five to nine days and four to six weeks following therapy. One patient in each group was unsuccessfully treated. Relapse or reinfection occurred with similar frequency in both treatment groups. Both antibiotics were well tolerated. minimal inhibiting concentrations (MICS) of cefaclor, amoxicillin, cephradine, and cephalexin were determined by an agar dilution technique for the 44 available pretreatment isolates (41 Escherichia coli and 3 Proteus mirabilis). Mean MICs (micrograms./ml., +/- SD) were 2.2 +/- 1.4 for cefaclor, 4.4 +/- 2.0 with amoxicillin, 8.1 +/- 4.2 for cephradine, and 5.7 +/- 3.0 with cephalexin. Cefaclor is highly active in vitro against those gram-negative bacteria which are commonly isolated from urine. Cefaclor is as effective as amoxicillin when administered three times daily for the treatment of urinary tract infection.