Natamycin is a tetraene polyene antibiotic derived from Streptomyces natalensis. Natamycin probably exerts its antifungal effects by binding to sterols in the fungal cell membrane to produce a change in membrane permeability that allows loss of essential cellular constituents. It possesses in vitro activity against a variety of yeast and filamentous fungi, including Candida, Aspergillus, Cephalosporium, Fusarium and Penicillium. Although the activity against fungi is dose-related, Natamycin is predominantly fungicidal. Topical administration appears to produce effective concentration of Natamycin within the corneal stroma but not in intraocular fluid.