The kinetics of the antibacterial activity of zosterin, a polysaccharide preparation of a sea grass belonging to Zoster, was studied. By its chemical structure zosterin is a low ++methoxylated pectin. In vitro the preparation markedly inhibited the growth of ++Gram-negative and ++Gram-positive organisms: S. aureus, E. coli, Y. pseudotuberculosis, S. typhimurium and Ps. aeruginosa. On a model of experimental pseudotuberculosis++ infection caused by oral contamination of mice F1 (CBA X C57B1) with a suspension of Y. pseudotuberculosis zosterin was shown to have a therapeutic effect. It protected 30 to 40 per cent of the animals when administered per os simultaneously with or 24 hours after the contamination. The results are in favour of the zosterin further investigation as a preparation useful in prevention of intestinal infections in persons being in contact with the patients.