AbstractWe have isolated a high molecular weight compound from tobacco callus (Nicotiana tabacum) that is active in lowering intraocular pressure (IOP) when injected intravenously into rabbits. Extracts were made from 2‐week‐old callus and from callus maintained in culture for varying amounts of time. Extracts of new callus were tested for IOP‐lowering activity and analysed for carbohydrate, protein, amino acid composition, and uronic acid content. Initial studies showed these extracts to be very active, lowering IOP almost to the maximum at doses of 200 μg. Partial characterization of the crude extract indicated that it contained 22% protein, 12% neutral sugar, and 5% uronic acid by weight. Extracts from callus maintained in culture for 2 months showed marginal activity when tested at 200 μg per animal. The material was fractionated by gel permeation chromatography on Fractogel TSK HW 65F. Fractions from chromatography were analysed for neutral sugar, protein, neutral sugar composition, and uronic acid content. The composition of the active material resembles plant cell wall components of the pect