T he administration of the immunosuppressive humanized monoclonal antibody CAMPATH 1-H, which recognizes CD52 on lymphocytes and monocytes, is associated with a first-dose cytokine-release syndrome involving TNFalpha, IFNgamma, and IL-6 clinically. In vitro models have been used to establish the cellular source and mechanism responsible for cytokine release, demonstrating that cytokine release is isotype dependent, with the rat IgG2b and human IgG1 isotype inducing the highest levels of cytokine release, which was inhibited with antibody to CD16, the low affinity Fc-receptor for IgG (FcgammaR). Cross-linking antibody opsonized CD4 T lymphocytes failed to stimulate TNFalpha release, which together with the observation that TNFalpha release by purified natural killer (NK) cells stimulated by fixed autologous CAMPATH 1-H-opsonized targets was inhibited with anti-CD16, indicates that cytokine release results from ligation of CD16 on the NK cells, rather than Fc-receptor (FcR)-dependent cross-linking of CD52 on the targeted cell. Since the hierarchy of isotypes inducing cytokine release in these cultures matches that seen clinically, we conclude that ligation of CD16 on NK cells is also responsible for cytokine release after injection of CAMPATH 1-H in vivo.