Until present studies are lacking which investigate the bactericidal activity of new quinolones in body fluids. Therefore, we determined bactericidal titers of enoxacin (en) and ciprofloxacin (cip) against a typical pathogen in urinary tract infections (Escherichia coli) in urine and against a typical pathogen in respiratory tract infections (Streptococcus pyogenes) in sputum, in each case at the time of peak and trough levels (In vitro data of the test strains: E. coli - MICen = 0.06 mg/l, MICcip = 0.06 mg/l, MBCen = 0.5 mg/l, MBCcip = 0.25 mg/l; Streptococcus pyogenes - MICen = 32 mg/l, MICcip = 1 mg/l, MBCen greater than 64 mg/l, MBCcip greater than 64 mg/l). Following a randomization list, ten healthy volunteers took either 400 mg enoxacin b.i.d. for three days, then (after a break of at least three days) 500 mg ciprofloxacin b.i.d. for three days, or vice versa. Two and 12 hours after the final dose, samples of sputum were taken, urine was collected 2-4 h and 10-12 h after the final dose. The bactericidal titers against E. coli in urine were greater than 1:512 (2-4 h after the final dose) and greater than 1:64 (10-12 h after the final dose) for both quinolones in all cases. On the other hand, as to S. pyogenes were found growth in every dilution of sputum. These results confirm the scepticism against quinolone therapy of respiratory tract infections caused by streptococci.