A naturally-occurring human IgM monoclonal antibody with potential antineoplastic activity. Human monoclonal antibody 216, derived from the gene VH4-34, binds to the glycosylated epitope CDIM on the surface of both malignant and normal B cells. Upon binding to B cells, this antibody may crosslink two or more CDIM molecules, resulting in the formation of cell membrane pores, the disruption of cell membrane integrity, and B cell lysis; this mechanism of antibody-mediated cell death is direct and does not involve mechanisms of complement-mediated cytotoxicity or antibody-depndent cell-mediated cytotoxicity (ADCC). CDIM is the glyco-moiety of a 75 kD MW B-cell cell surface glycoprotein.