Mucopolysaccharide (YMP) of yam (Dioscorea batatas Decne) was tested for immunomodulatory activity in vitro. We examined the effect of YMP on the viability, cell-mediated cytotoxicity and IFN-γ secretion of splenic lymphocytes. The cytotoxic activity of mouse splenocyte against leukemia cell was increased in the presence of YMP (10μg/ml). However, YMP (10μg/ml) did not affect the viability of splenocytes. The production of IFN-γ was significantly increased in the YMP treated splenocytes, suggesting that YMP may induce cell-mediated immune responses. The effects of YMP on phagocytic activity were studied in relation to uptaking capacity and lysosomal phosphatase activity of the mouse peritoneal macrophages in vitro. YMP (50μg/ml) was found to increase uptaking capacity and lysosomal phosphatase activity of peritoneal macrophages. In addition, YMP (10-100μg/ml) significantly increased the viability of peritoneal macrophages (P<0.05). The data obtained demonstrate that the YMP may be used as sources of immunoactive polysaccharides.