A newly identified member of the tumor necrosis factor receptor (TNFR) superfamily shows activities associated with osteoclastogenesis inhibition and fibroblast proliferation. This new member, called TR1, was identified from a search of an expressed sequence tag database, and encodes 401 amino acids with a 21‐residue signal sequence. Unlike other members of TNFR, TR1 does not contain a transmembrane domain and is secreted as a 62 kDa glycoprotein. TR1 gene maps to chromosome 8q23–24.1 and its mRNA is abundantly expressed on primary osteoblasts, osteogenic sarcoma cell lines, and primary fibroblasts. The receptors for TR1 were detected on a monocytic cell line (THP‐1) and in human fibroblasts. Scatchard analyses indicated two classes of high and medium‐high affinity receptors with a kD of approximately 45 and 320 pM, respectively. Recombinant TR1 induced proliferation of human foreskin fibroblasts and potentiated TNF‐induced proliferation in these cells. In a coculture system of osteoblasts and bone marrow cells, recombinant TR1 completely inhibited the differentiation of osteoclast‐like multinucleated cell formation in the presence of several bone‐resorbing factors. TR1 also strongly inhibited bone‐resorbing function on dentine slices by mature osteoclasts and decreased ⁴⁵Ca release in fetal long‐bone organ cultures. Anti‐TR1 monoclonal antibody promoted the formation of osteoclasts in mouse marrow culture assays. These results indicate that TR1 has broad biological activities in fibroblast growth and in osteoclast differentiation and its functions.—Kwon, B. S., Wang, S., Udagawa, N., Haridas, V., Lee, Z. H., Kim, K. K., Oh, K‐O., Greene, J., Li, Y., Su, J., Gentz, R., Aggarwal, B. B., Jian, Ni, J. TR1, a new member of the tumor necrosis factor receptor superfamily, induces fibroblast proliferation and inhibits osteoclastogenesis and bone resorption. FASEB J. 12, 845–854 (1998)