Human ribonuclease inhibitor (hRI) is an acid protein with a molecular weight of 50 kDa. It can inhibit the activity of pancreatic RNase (RNase A). Angiogenin (Ang) is a member of the ribonuclease super family. It has 35% identity with RNase A and contains ribonucleolytic activity. The substrate specificity of angiogenin seems, however, to be more restricted than that of the pancreatic RNase. Since Ang is an important angiogenic factor and RI is a highly efficient inhibitor of Ang, it can be hypothesized that RI may be a latent antiangiogenic drug. This study focuses on the feasibility of transfecting the ri gene into mice hematopoietic cells and inducing the expression of the ri gene to block the angiogenesis of solid tumors. First, the cDNA gene of the ri from human placenta was cloned and inserted in a retroviral vector, pLNCX. The combined vector pLNCX-ri was transfected into retroviral packaging cells, PA317, and a clone producing a high titer of virus was obtained. Next, isolated hematopoietic cells from mice bone marrow were infected with viruses carrying the pLNCX-ri. The infected cells were then injected into lethally irradiated mice. The expression and the contribution of RI were assayed in vivo. After administration of hematopoietic cells carrying the ri gene, mice were implanted with B16 melanomas for 21 days. The results showed that tumors of control groups became large and well vascularized. In contrast, tumors from mice groups treated with hematopoietic cells carrying the ri gene were small and possessed a relatively low density of blood vessels. The inhibited growth rate of the tumors was 47%. This study demonstrated the potential utility of gene therapy for systemic delivery of a novel antiangiogenic agent--hRI.