We have investigated the capacity of polymorphonuclear phagocytes (PMN) to lyse malignant B-cell lines using antibodies and antibody derivates to a range of different B-cell antigens. PMN were found to mediate lysis of all tested B-cell lines in the presence of HLA class II antibodies L227, L243, F3.3, and CR3/43. Target cell lysis was significantly enhanced when PMN isolated during granulocyte colony- stimulating factor (G-CSF) treatment were compared with PMN from healthy donors. Only G-CSF primed PMN, expressing Fc gamma RI (CD64), lysed B cells in the presence of monoclonal antibody (MoAb) 1D10 or Lym- 1 to HLA class II related epitopes. Remarkably, PMN were consistently unable to kill malignant B cells with antibodies to the B-cell related antigens CD19, CD20, CD21, CD37, and CD38. These target antigen restriction was not observed with mononuclear effector cells, which mediated cytotoxicity with antibodies to HLA class II, but also with mouse/human chimeric constructs to CD19, CD37, and CD38. Blocking studies with Fc gamma RI antibodies and reverse antibody-dependent cellular cytotoxicity (ADCC) experiments against Fc gamma R antibody expressing hybridoma targets confirmed the pivotal role of Fc gamma RI in enhanced killing by G-CSF primed neutrophils. Bispecific antibodies (BsAb) with one specificity for Fc gamma RI, and another for a tumor associated antigen, offer an interesting approach to improve effector cell recruitment for immunotherapy. In our studies, very effective lysis was observed with G-CSF primed PMN and an [HLA class II x Fc gamma RI] BsAb. The therapeutic implications of these findings and the possible use of BsAb in combination with G-CSF are discussed.