Bone marrow stromal cells regulate B-cell growth and development through their surface molecules and cytokines. In this study, we generated a mAb, RS38, that recognized a novel human membrane protein, BST-2, expressed on bone marrow stromal cell lines and synovial cell lines. We cloned a cDNA encoding BST-2 from a rheumatoid arthritis-derived synovial cell line. BST-2 is a 30- to 36-kDa type II transmembrane protein, consisting of 180 amino acids. The BST-2 gene (HGMW-approved symbol BST2) is located on chromosome 19p13.2. BST-2 is expressed not only on certain bone marrow stromal cell lines but also on various normal tissues, although its expression pattern is different from that of another bone marrow stromal cell surface molecule, BST-1. BST-2 surface expression on fibroblast cell lines facilitated the stromal cell-dependent growth of a murine bone marrow-derived pre-B-cell line, DW34. The results suggest that BST-2 may be involved in pre-B-cell growth.