Gram-positive cocci are the most common pathogens in severe human eye infections. Streptococcal endophthalmitis is a devastating infection, and intravitreal antibiotic therapy is limited by retinal toxicity. Because few systemic antistreptococcal antibiotics penetrate into the vitreous, sparfloxacin, a newer quinolone with improved antistreptococcal activity, might be of interest. We therefore assessed its efficacy by the intravitreal route in a rabbit model of streptococcal endophthalmitis. The vitreal bacterial count (mean +/- standard deviation log10 CFU per milliliter) was significantly reduced after an intravitreal injection of 800 micrograms of sprafloxacin (4.9 +/- 0.7) relative to the counts in untreated control (7.1 +/- 0.7) and pefloxacin-treated (7.8 +/- 1.2) eyes. After systemic administration to rabbits, the maximum concentration of sparfloxacin in serum was 5.6 micrograms.ml-1 and the half-life was 7.5 h. Sparfloxacin exhibited very good penetration ratios in the vitreous (54%), cornea (76%), and lens (36%). In the vitreous, the levels of sparfloxacin remained greater than the MICs for most gram-positive cocci for up to 18 h. Further experimental studies are warranted to determine the efficacy of systemic sparfloxacin as adjuvant therapy in the treatment of human endophthalmitis.