AbstractTrichosanthin (TCS) is a ribosome-inactivating protein from root tubers ofTrichosanthes kirilowiiMaxim. In this paper, the effects of TCS on the viability of human peripheral blood immunocytes, on the proliferation of lymphocytes, and its cytotoxicity to twelve cell lines of lymphoma or leukemia had been observed. TCS at high concentration (>12.5μg/ml) affected the viability of human B lymphocytes, but not that of human peripheral blood mononuclear cells (PBMCs), T lymphocytes and granulocytes. Human peripheral blood-derived monocytes/macrophages were hrghly sensitive to TCS (ID50 at 1.70μg/ml). TCS suppressed lymphocyte proliferation stimulated by Concanavalin A (Con A) or lipopolysaccharide (LPS). Human T cell lines and macrophage cell lines were more sensitive (ID50<0.9μg/ml) to TCS than B cell lines and myeloid lines. These results suggest that selective cytotoxicity of TCS to human macrophages/monocytes may be implicated in anti-HIV activity, and that selectively killing some leukemia-lymphoma cells by TCS merit further evaluation in treatment of some lymphoma and leukemia.