The effect of dietary fat on the immunotoxicity of chloramphenicol was investigated in mice, sensitized and challenged with sheep red blood cells. Chloramphenicol suppressed more cell-mediated immunity than humoral immunity. The saturated fat diet elevated humoral and cell-mediated immunotoxicity, whereas the unsaturated fat diet decreased humoral immunotoxicity, but elevated cell-mediated immunotoxicity of chloramphenicol. Especially, the normal diet in saturated and unsaturated fat restored immunosuppressive effect of chloramphenicol compared to the saturated fat diet and the unsaturated fat diet.