Studies on bacterial elimination in urine were done by using the membrane fIltration technique on patients with UTI during treatment with several β-lactam antibiotics and co-trimoxazole. Duringβ-lactam antibiotic therapy the decrease of colony count was faster than with co-trimoxazole within the fIrSt 4 h. After 24 h many more samples from patients treated with bacampicillin or amoxycillin were sterile than those treated with co-trimoxazole. Five days later no differences were seen between the groups. Cefotaxime, a new cephalosporin derivative, gave a more rapid pathogen elimination than any other antimicrobial agent. Bacterial elimination in the urine during treatment is a useful method for evaluating the effIcacy of antibiotics in patients with UTI.