Fucithalmic is a newly developed 1% microcrystalline suspension of the antistaphylococcal antibiotic fusidic acid. Penetration into the eye was investigated in twenty patients about to undergo cataract extraction. A single dose produced median antibiotic levels in the aqueous humour of 0.3 mcg/ml, maintained for at least 12 hours. Repeated doses produced significantly higher levels, median 0.8 mcg/ml, indicating some cumulative effect. The results show that fusidic acid passes the corneal aqueous barrier, and twice daily administration of Fucithalmic gives aqueous concentrations comparable with or higher than those seen after recommended systemic administration.