Rice (Oryza sativa L.) is a staple food worldwide for centuries. In this study, the growth-inhibiting and immunopotentiating effects of commonly used rice were examined. The growth of human leukemic U937 cells was significantly inhibited by the peripheral blood mononuclear cell-conditioned medium (MNC-CM) derived from water extracts of Japonica rice milled Taiwan 9 (MT9) and brown Taiwan 9 (BT9). Furthermore, these MNC-CMs induced differentiation of U937 cells into mature monocytes/macrophages expressing superoxide-producing and phagocytic activity. The amounts of tumor necrosis factor-alpha and interferon-gamma in MNC-CMs prepared with MT9 and BT9 were greater than normal MNC-CM. However, growth of U937 cells was not inhibited by normal MNC-CM or rice extracts alone. The possible active components of MT9 and BT9, other than PHA-like glycoproteins and heat-sensitive proteins and lipopolysaccharides, remain to be determined. Our results demonstrate that MT9 and BT9 can inhibit growth and induce differentiation of leukemic U937 cells through activation of human peripheral blood MNC in vitro. These two types of rice as energy-providing food could be biological response modifiers for augmenting anti-leukemia immunity.