Tumor-associated cell surface antigens and tumor-associated vascular markers have been used as a target for cancer intervention strategies. However, both types of targets have limitations due to accessibility, low and/or heterogeneous expression, and presence of tumor-associated serum antigen. It has been previously reported that a mitochondrial/cell surface protein, p32/gC1qR, is the receptor for a tumor-homing peptide, LyP-1, which specifically recognizes an epitope in tumor cells, tumor lymphatics, and tumor-associated macrophages/ myeloid cells. Using antibody phage technology, we have generated an anti-p32 human monoclonal antibody (2.15). The 2.15 antibody, expressed in single-chain fragment variable and in trimerbody format, was then characterized in vivo using mice grafted subcutaneously with MDA-MB-231 human breast cancers cells, revealing a highly selective tumor uptake. The intratumoral distribution of the antibody was consistent with the expression pattern of p32 in the surface of some clusters of cells. These results demonstrate the potential of p32 for antibody-based tumor targeting strategies and the utility of the 2.15 antibody as targeting moiety for the selective delivery of imaging and therapeutic agents to tumors.The localization of tumors may be accomplished by any of several combinations including computed tomography, ultrasonography, gamma camera examination, and glucose consumption (1, 2). However, targeted localization of the tumors is preferred, mainly using specific probes that bind to tumorassociated cell surface antigens or to markers of angiogenesis expressed by endothelial cells or present in the surrounding extracellular matrix (3-6). Probes that bind to tumor-associated cell surface antigens have some drawbacks (7) such as the heterogeneous expression on the cell surface or the increased serum levels of the antigen as tumors grow, which may act as a trap for the targeting agent. Angiogenesis related targets are readily accessible; however, the relatively low abundance of endothelial cells in tumor tissue makes the molecular imaging of tumor neovessels more challenging. Furthermore, angiogenesis may occur also in a physiological context, thus adding more complexity to the targeting.With these limitations in mind, we hypothesized as an alternative target a marker selectively expressed in different compartments in the tumor area. One targeting agent specific for the tumor but not restricted to the tumor cells is the tumor homing peptide (LyP-1), which strongly and specifically accumulates in the tumor after systemic administration, localizing preferentially associated to lymphatic markers (8 -10). LyP-1-binding protein was characterized as p32 (10), a multiligand and multicompartmental protein that has been independently identified in several contexts and has been named accordingly as SF2P32 (splicing factor SF2-associated protein; 11), HABP-1 (hyaluronic acid binding protein-1; 12), gC1qR (globular domain of C1q receptor; Ref. 13), or HIV TAP (Tatassociated protein; 14). Although p...