The tolerant-selenium exopolysaccharide-producing bacterial strain Enterobacter cloacae 20206 was batch cultured in PDA medium containing optimal concentration of sodium selenite. Selenium was accumulated efficiently in Enterobacter cloacae 20206 during cultivation with selenium. Inorganic selenite could be transformed into organic forms. Selenium-enriched exopolysaccharide (Se-ECZ-EPS-1) was purified from the fermentation liquid. Selenium content of Se-ECZ-EPS-1 was 12.962 mu g/g. Se-ECZ-EPS-1 with Mw of 29,300 Ka was composed of Glc, Gal and Mann with molar ratio of 8.530:0.061:0.706. Administration of Se-ECZ-EPS-1 to cyclophosphamide (CP)-exposed animals resulted in improvement of cellular and humoral immune responses. These findings indicated that Se-ECZ-EPS-1 may act as potent immunomodulatory agents. Crown Copyright (c) 2008 Published by Elsevier Ltd. All rights reserved.