Spent brewer's yeast was autolysed and the insoluble yeast cell wall fraction obtained was used as a raw material for the preparation of brewer's yeast beta-glucan (BYG). A simple alkaline extraction was applied and optimized. The BYG with significantly (P</=0.05) high carbohydrate and glucan contents of 92% and 51% (w/w), respectively and with a low protein content of 1.6% (w/w) was obtained by a single alkaline extraction using 5 volumes of 1.0 N NaOH at 90 degrees C for 1 h. In vitro, this glucan significantly (P</=0.05) enhanced phenoloxidase (PO) activity of black tiger shrimp hemolymph as compared to controls without added glucan. Also in vivo, an oral administration of 0.2% (w/w) in diets for 3 days significantly (P</=0.05) increased the PO-activity of the shrimp. The potential immunostimulating properties of a commercial and other yeast-derived products were also evaluated for comparative purposes.