Neutrophils mediate the lysis of human neuroblastoma cells coated with human/mouse chimeric anti-GD2 ganglioside antibody ch14.18. This study examined the mechanism(s) by which this occurs. Neutrophil degranulation was found to be a required step for lysis, since release of granular enzymes from neutrophils correlated with the lysis of antibody-coated neuroblastoma cells. In addition, agents which block degranulation specifically inhibited this process. Antibody-dependent lysis of neuroblastoma cells was enhanced by exposing neutrophils to granulocyte-macrophage colony stimulatory factor. An increased release of lytic granular molecules was found to be responsible for this lymphokine-mediated phenomenon. Among the molecules released from neutrophil granules that were shown to be involved in neuroblastoma cell lysis were defensins, M(r) 3000-4000 neutrophil granular proteins which are known to bind and permeabilize tumor cells. In addition, cathepsin-G, a neutrophil granular protease, was demonstrated for the first time to mediate the lysis of human neuroblastoma cells. The enzymatic activity of cathepsin-G was found to be required for the lysis of these tumor cells, since phenylmethylsulfonyl fluoride blocks the lytic ability of this protein.