The mucin-type sialoglycoprotein podoplanin (aggrus) is involved in tumor cell-induced platelet aggregation and tumor metastasis. C-type lectin-like receptor-2 (CLEC-2) was recently identified as an endogenous receptor of podoplanin on platelets. However, the pathophysiological importance and function of CLEC-2 have not been elucidated. Here we clarified the pathophysiological interaction between podoplanin and CLEC-2 in vitro and in vivo. Using several deletion mutants of CLEC-2 expressed as Fc chimeras, we first identified an important podoplanin-recognition domain in CLEC-2. Furthermore, the podoplanin-CLEC-2 interaction was confirmed using several deletion mutants of podoplanin expressed as Fc chimeras. Not only the disialyl-core1-attached glycopeptide but also the stereostructure of the podoplanin protein was found to be critical for the CLEC-2-binding activity of podoplanin. We next synthesized various glycopeptides of podoplanin that included both the platelet aggregation-stimulating domain and O-glycan on Thr52. Interestingly, a disialyl-core1-attached glycopeptide was recognized specifically by CLEC-2. Moreover, the anti-podoplanin monoclonal antibody NZ-1 suppressed both the podoplanin-CLEC-2 interaction and podoplanin-induced pulmonary metastasis, suggesting that CLEC-2 is the first pathophysiological receptor of podoplanin to be identified. In summary, we clarified the molecular interaction in.vitro and in.vivo between a platelet aggregation-inducing factor, podoplanin, and its specific pathophysiological receptor on platelets, CLEC-2. Podoplanin and CLEC-2 might represent promising therapeutic targets in cancer metastasis.