The antibacterial activity of ciprofloxacin was compared to those of norfloxacin, pefloxacin, pipemidic acid, nalidixic acid, nitrofurantoin, sulfamethoxazole, trimethoprim, cephradine and amoxycillin. Agar dilution tests were performed with 631 clinical isolates from urinary and respiratory tract infections. Ciprofloxacin was found to be the most active drug tested against all gram-negative organisms and streptococci, with the exception of Streptococcus faecalis and Streptococcus pneumoniae. MIC 90 values of ciprofloxacin were as follows: for Enterobacteriaceae, 0.03-0.23 mg/l, Pseudomonas aeruginosa, 0.37 mg/l, Haemophilus influenzae, less than 0.015 mg/l, Staphylococcus aureus, 0.75 mg/l, Streptococcus pneumoniae, 1.89 mg/l, and Streptococcus faecalis, 0.95 mg/l. The inhibitory quotients for urine, serum and bronchial secretion showed that ciprofloxacin had the broadest spectrum of all agents tested and covered and clinically significant bacteria.