A proprietary antibody-based single-chain polypeptide with B cell-depleting activity. Significantly smaller than a whole antibody, CD20-targeted polypeptide TRU-015 binds specifically to the B cell-specific cell surface antigen CD20 with full immunoglobulin Fv fragment-type target binding activity and full immunoglobulin Fc fragment-type effector function. This agent transiently depletes CD20-bearing B cells by inducing B cell -directed complement dependent cytotoxicity (CDC) and antibody-dependent cell-mediated cytotoxicity (ADCC) and B cell apoptosis.