A radioimmunoconjugate consisting of the murine IgG1 anti-CD19 monoclonal antibody (MoAb) BU12 labeled with the beta-emitting radioisotope yttrium Y 90 with radioisotopic and antibody activities. Yttrium Y 90 anti-CD19 monoclonal antibody BU12 binds to the CD19 molecule, specifically delivering cytotoxic beta radiation to CD19-expressing B cells. CD19 is a membrane antigen that is widely expressed during B-cell development, from pro-B-cell to early plasma cell stages.