Objective To define roles of B7-1 co-stimulation factor expressed in human malignant cell lines in mediating anti-tumor T cell immune responses. Methods Examining human leucocyte antigen (HLA) and B7 expressions on 8 human blood malignancies cell lines by flow cytometry. Transfecting B7-1 gene to B7-1 negative (B7*!-) Raji and B7*!- Jurkat cell lines by liposome, and comparing the potencies of blood malignant cell lines in the induction of T cell activation by examination of T cell cytokine mRNAs before and after transfection using semi-quantitative reverse transcription polymerase chain reaction (RT-PCR). Results High level of HLA Ⅰ and Ⅱ molecules were expressed in most human blood malignant cell lines examined, and the co-stimulatory factor B7-2 was also highly expressed. In contrast, another member of B7 family: B7-1 was either not expressed or very limitedly expressed in most of these hematopoietic malignant cell lines. Most importantly, transfection of B7-1 gene to B7*!-. Raji and B7*!-. Jurkat cell lines made these cell lines better antigen presenting cells for stimulation of anti-tumor T cell activation, which was demonstrated by up regulation of expression of T cell cytokines IL-2, IL-4 and INF-γ mRNAs after incubation of these tumor cells with T cells for 24 h. Conclusions B7 co-stimulation plays an important role in anti-tumor immunity. Transfection of B7-1 gene to the human hematopoietic malignant cell lines that are deficient in the B7-1 expression empowers their antigen presentation potency for activation of anti-tumor T cells. Our results suggested that repairing the deficiency of B7-1 co-stimulatory pathway in tumor cells might be a novel immunotherapeutic approach for human hematopoietic malignancies.