We previously reported Frizzled homolog 10 (FZD10), a member of the Frizzled family, to be a promising therapeutic target for synovial sarcomas. In this report, we established a murine monoclonal antibody (MAb), namely, MAb 92-13 that had specific binding activity against native FZD10 product expressed in synovial sarcoma cell lines. Subsequent immunohistochemical analyses with the MAb 92-13 confirmed an absence or hardly detectable level of FZD10 protein in any normal human organs. We confirmed the specific binding activity of this MAb in vivo after injection of fluorescent-labeled MAb i.p. or i.v. into the mice carrying synovial sarcoma xenografts by the use of the in vivo fluorescent imaging system as well as radioisotopes. Moreover, MAb 92-13 was effectively internalized into the synovial sarcoma cells after its binding to FZD10 on the cell surface. A single i.v. injection of the Yttrium-90 ((90)Y)-MAb 92-13 drastically suppressed tumor growth of synovial sarcoma in mice without any severe toxicity. Median time to tumor progression was 58 days for mice treated with (90)Y-MAb 92-13 and 9 days for mice treated with non-labeled antibody control or untreated mice (difference = 49 days; P = 7 x 10(-5)). This result indicates that MAb 92-13 could be utilized as the novel treatment modality for synovial sarcoma and other FZD10-positive tumors.