Annual change in bacteria detected in urinary infections and their sensitivity to antibacterial agents in 3-year drug history was studied for selection of the drugs of choice. Grampositive bacteria decreased in the 3-year period, while gram-negative organisms (mostly E. coli, Klebsiella, Proteus, P. aeruginosa, Enterobacter) increased markedly. E. coli increased resistance to AB-PC, Klebsiella to CEZ, and P. aeruginosa to GM; and E. coli, Klebsiella and Enterobacter increased sensitivity to CP. The drugs of first choice thus selected for controlling the 5 species of bacteria were: CEX, NA, CEZ and GM for E. coli; CEX, NA and CEZ for Klebsiella; CB-PC, AMK and SB-PC for P. aeruginosa; NA, TC and GM for Enterobacter. Then, sensitivity to CB-PC and SB-PC disks of P. aeruginosa and Proteus detected in the urine samples in 3 years was compared. Proteus showed no difference in sensitivity to the two agents, while P. aeruginosa was more sensitive to SB-PC than to CB-PC. It is, therefore, necessary to study the factors in the difference and to compare clinical usefulness of the two drugs.