Abstract： Objective To study the selective toxicity of immunotoxin (IT) onT cells in cord blood and simultaneously determine its effect on hematopoietic progenitor cells. Methods The percentage of CD5 and CD8 T cell subsets in cord blood (CB) and bone marrow (BM) as well as peripheral blood (PB) was measured by immunoenzymatic labeling of monoclonal antibodies using immune complexes of alkaline phosphatase and monoclonal anti-alkaline phosphatase (APAAP complexes). One-way mixed lymphocyte cultures (MLC) were performed to compare the proliferative response of CB with that of PB. The proliferative capability of cord blood T cells and T lymphocyte transformation capacity were evaluated in the presence of anti-CD8 or anti-CD5 immunotoxin by one-way MLC and colorimetric MTT (tetrazolium) assay, respectively. The effect of IT on the growth of hematopoietic progenitor cell of colony forming unit-granulocyte and macrophage (CFU-GM), burst forming unit-erythroid(BFU-E), multipotential hemotapoietic progenitors (CFU-Mix) from CB were estimated by colony-forming assays. Results A certain proportion of CD5 and CD8 T cells existed in CB. The alloproliferative capacity of CB was similar to that of PB. CD5: Ricin at a dosage of 1×10-10-1×10-8 mmol/L and CD8: Ricin concentration in the range of 1×10-9-1×10-8 mmol/L effectively decreased both the proliferative capability of T cells in MLC during CB and T cell transformation. Over the dosage of 1×10-10-1×10-9 mmol/L, both kinds of IT didn't obviously affect the growth of hematopoietic progenitor cells. Conclusion CD5: Ricin and CD8: Ricin may effectively deplete T cells and may not significantly inhibit the function of hemaptopoietic cells at a specific dosage.