Clarithromycin has a wide spectrum of activity against many gram-positive and gram-negative organisms, intracellular pathogens, and opportunistic pathogens. To examine the penetration of clarithromycin in the ocular tissues, 21 patients who underwent elective cataract surgery (Group I) received a single 500-mg dose of clarithromycin orally either 4, 8, 10, 12, or 22 hours before cataract surgery, and 21 patients who underwent elective retina/vitreous surgery (Group II) received 500 mg every 12 hours orally for 3 days before the surgery with the last dose given either 3, 6, 8, 11, or 24 hours before the surgery. Serum from all patients was assayed for clarithromycin prior to drug administration and at the time ocular specimen was taken. Aqueous, iris, and vitreous samples were also assayed for clarithromycin concentration. The concentrations of clarithromycin in the aqueous fluid 4, 8, 10, 12, and 22 hours after administration were: (mean +/- SD) 0.13+/-0.05, 0.137+/-0.11, 0.074+/-0.03, 0.06+/-0.02, and 0.074+/-0.04 microg/ml, respectively. Concentration of clarithromycin in vitreous 3, 6, 8, 11, and 24 hours after administration were: (mean +/- SD) 0.11+/-0.02, 0.257+/-0.13, 0.27+/-0.21, 0.307+/-0.26 and 0.108+/-0.07 microg/ml, respectively. The mean concentration of clarithromycin in the iris was 6.2 microg/g. In conclusion, this data suggest that clarithromycin widely penetrates and adequately concentrates in the aqueous humor, vitreous humor, and iris tissue after oral administration and therefore is effective in the management of many infectious ocular conditions.