We investigated the inhibition of proliferation of human retinal pigment epithelial cells in vitro by the 4-quinolone, ciprofloxacin, and the steroid, dexamethasone. The concentration of ciprofloxacin that inhibited growth by 50% (IC50) was found to be 14.1 micrograms/mL. Growth was 100% inhibited at 83 micrograms/mL. At 166 micrograms/mL, all the cells became completely detached and appeared dead at the end of seven days. The IC50 for dexamethasone in RPE cells was found to be 141 micrograms/mL. A dexamethasone concentration of 1.3 mg/mL inhibited proliferation 100% after five days. When the two drugs were combined, the inhibitory effect was found to be additive; i.e., the IC50 dose of the two drugs in combination inhibited RPE cell proliferation by 75%. A combination of the two drugs was also tested for retinal toxicity in rabbit eyes. An examination of histological sections and electroretinograms showed that a dose of 100 micrograms of ciprofloxacin, alone or in combination with 200 micrograms of dexamethasone in saline, was not toxic to the rabbit retina. These studies indicate that a combination of ciprofloxacin and dexamethasone has the potential for reducing the risk of PVR formation and aiding in the prevention of endophthalmitis.