The in vitro activity of sparfloxacin, a new quinolone, was compared with those of ciprofloxacin and fleroxacin against gram-positive and gram-negative bacteria, >90% of which were isolated from blood culture specimens of cancer patients. Sparfloxacin was extremely active against Acinetobacter species, Aeromonas hydrophila, Citrobacter diversus, Enterobacter species, Escherichia coli, Kiebsiella species, Proteus vulgaris, and Serratia marcescens (inhibiting >90% of these isolates at a concentration of 0.5 ,ig/ml) and moderately active against Pseudomonas species, other Proteus species, and Citrobacterfreundii. Sparfloxacin inhibited >90% of staphylococci (including methicillin-resistant and coagulase-negative strains) at a concentration of 0.12 ,ig/ml and >90% of streptococci (including Streptococcus pneumoniae) at a concentration of 1.0 ,ug/mI. It was also active against Bacillus cereus, Enterococcus species, and Corynebacterium jeikeium, organisms that have become fairly common in cancer patients.New quinolones, such as norfloxacin, ciprofloxacin, and ofloxacin, are currently being used for both prophylaxis and therapy of infections in neutropenic cancer patients (1-6, 9, 14, 16). They are active against most gram-negative pathogens, but their activity against gram-positive organisms, particularly Streptococcus and Enterococcus species, is inadequate. The prophylactic use of 4-quinolones in patients with leukemia has resulted in a decrease in the number of documented gram-negative infections (5, 6, 16). The use of these agents, however, either has had no effect on grampositive infections or has occasionally led to an increase in such infections (2). Newer 4-quinolones that are active against commonly occurring gram-negative and gram-positive pathogens are therefore desirable. Sparfloxacin (CI-978; AT-4140) is a new [5-amino-1-cyclopropyl-6,8-difluoro-1,4-dihydro-7-(cis-3,5-dimethyl-1-piperazinyl)-4-oxoquinoline-3-carboxylic acid] quinolone that is reported to have a broad antibacterial spectrum (7,11). We compared its in vitro activity to those of ciprofloxacin and fleroxacin.The