We studied the activity of ciprofloxacin and other antibiotics against both routine and multiresistant (multi-R) clinical isolates. Ciprofloxacin inhibited more than 98% of most species of Enterobacteriaceae at a concentration of 2 micrograms/ml. Only Acinetobacter calcoaceticus, and to a much lesser degree, Providencia and Serratia, were resistant. Most Pseudomonas aeruginosa isolates were susceptible. Only 1% of staphylococci were resistant; the test panel included 1200 MRSA. For most species of streptococci, the MIC90 was 1 microgram/ml; for enterococci, it was 2 micrograms/ml. We also surveyed resistance in our current isolates. Resistance to ciprofloxacin has increased in A. calcoaceticus and Providencia, and in Streptococcus pneumoniae and group B streptococci. Ciprofloxacin-resistant isolates tended to show increased resistance to other antibiotics, including aminoglycosides and, later, cephalosporins.