Lym-1 is a therapeutically promising IgG2a monoclonal antibody (MoAb) that reacts with variant class II molecules expressed on B-lineage malignancies. To optimize antitumor immunotherapy with Lym-1, we determined whether granulocytes could participate in tumor lysis mediated by Lym-1 and whether recombinant interferon gamma (IFN-gamma) could influence this reaction. Granulocytes had minimal activity in mediating Lym-1 antibody-dependent cellular cytotoxicity (ADCC) compared with peripheral blood mononuclear cells (PBMC). However, IFN- gamma markedly augmented and occasionally induced granulocyte Lym-1 ADCC in a dose-dependent fashion (optimal 100 U/mL). Granulocytes primed with IFN-gamma for 24 hours displayed greatly increased ADCC in the absence of further IFN-gamma exposure. In most individuals, IFN- gamma also enhanced PBMC Lym-1 ADCC. Interferon alfa (IFN-alpha), although able to enhance PBMC-mediated Lym-1 ADCC and non-ADCC lysis, had no effect on granulocyte activity. Lym-1 ADCC involved an interaction between Fc receptor-bearing granulocytes and Lym-1 antigen positive targets based on the following: (1) The reaction was Lym-1 dose-dependent with cytotoxicity detectable at concentrations as low as 0.5 microgram/mL. (2) An irrelevant, isotype-matched MoAb (UPC-10) was unable to mediate ADCC. (3) In the absence of Lym-1, granulocytes--with or without IFN-gamma--displayed no intrinsic tumor lytic ability. Conversely, without granulocytes, Lym-1 or Lym-1 plus IFN-gamma had no effect. (4) Protein A, which binds avidly to Lym-1, inhibited the reaction 95%. (5) IFN-gamma induced granulocyte expression of CD64, the IgG Fc receptor that binds murine IgG2a MoAbs. (6) Of nine malignant human cell lines, only the three that moderately or strongly expressed the Lym-1 antigen were consistently lysed. Preclinical studies such as these may provide a rational basis for designing clinical trials with Lym-1 in combination with IFN-gamma.