Pharmacologyonline 2: 22-40 (2008) Young Researchers Gardeva et al. 22 PROTECTIVE EFFECT OF DIXONIELLA GRISEA (RHODOPHYTA) POLYSACCHARIDE AGAINST MYELOID GRAFFI TUMOR IN HAMSTERS Elena Gardeva* 1 , Reneta Toshkova 1 , Tonka Toncheva-Panova 2 , Juliana Georgieva 2 , Ilina Krasteva 3 1 Institute of Experimental Pathology and Parasitology, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria (egardeva@abv.bg) * Author for correspondence; 2 Institute of Plant Physiology, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria; 3 Department of Pharmacognosy, Faculty of Pharmacy, Medical University, 2 Dunav St, Sofia 1000, Bulgaria Summary Immunomodulators of natural origin have raised a considerable interest in red microalga as a producer of high molecular weight polysaccharides because these biopolymers often show advantages over the polysaccharides that are currently in use. We investigated the protective effect of Dixoniella grisea polysaccharide (DgPSH) on Graffi myeloid tumor in hamsters (GTBH). Various doses and ways of application of polysaccharide administered before and after tumor transplantation were studied. The mean survival time, inhibition of tumor growth, mortality percent and transplantability were determined. We further investigated the in vitro effect of DgPSH on the functional state of peritoneal macrophages by macrophage spreading and phagocytosis. The obtained experimental data demonstrate the protective effect of Dixoniella grisea polysaccharide (DgPSH) on the survivability of GTBH as expressed by decrease of the tumor transplantability, inhibition of tumor growth, elongation of mean survival time and reduced mortality percentage. The DgPSH increases both spreading and phagocytic ability of peritoneal macrophages in healthy and GTBH. The stimulation followed a dose-dependent pattern. The observed protective effect at the beginning stage of tumor progression can be partly explained by the temporary immunostimulating and immunorestorating action of the polysaccharide on the immune cells (macrophages, lymphocytes, NK-cells) or by direct cytotoxicity on Graffi myeloid tumor. Studies aimed at elucidation of immunomodulating activities of red microalgal polysaccharides in Graffi tumor bearing hamsters are currently in progress. Key words: Dixoniella grisea, polysaccharide, Graffi myeloid tumor, macrophages.