The in vitro antimicrobial activity of ofloxacin, a new fluorinated quinolone, was evaluated against 165 gram-negative rods, both fermentative and non-fermentative and against 57 gram-positive strains (coagulase-positive and -negative, staphylococci both methicillin-resistant and susceptible and Streptococcus faecalis). Minimal inhibitory concentrations were determined by using the macrodilution test and activity was compared with nalidixic acid, norfloxacin, ampicillin, piperacillin, ceftazidime and gentamicin for gram-negative rods, norfloxacin and gentamicin for Staphylococcus strains, and norfloxacin, ampicillin, piperacillin and gentamicin for enterococci. Ofloxacin inhibited all fermentative gram-negative bacteria tested, in a range of 0.05-3.12 micrograms/ml. It also had good antimicrobial activity against non-fermentative gram-negative strains: in fact it inhibited 90% of Acinetobacter and 80% of Pseudomonas aeruginosa tested at 3.12 micrograms/ml. Ofloxacin had good antimicrobial activity against Staphylococcus and Enterococcus tested.