A previous study in dairy cows showed some effect of feed contaminated with Fusarium mycotoxins on metabolism and immunity. A subsequent experiment investigated the effect of feedborne Fusarium mycotoxins on some immune functions in more detail. A total mixed ration (TMR) containing a blend of feedstuffs naturally contaminated with Fusarium mycotoxins was fed for 63 d to 12 mid-lactation Holstein cows with an average milk production of 36 kg/d in a completely randomized design with repeated measures including 1) control TMR and 2) contaminated TMR. Wheat, corn, hay, and corn silage were the contaminated feedstuffs. Deoxynivalenol was the major contaminant and was found in TMR at 3.5 mg/kg of dry matter. The parameters measured were 1) performance: body weight, body condition score, dry matter intake, milk production, composition and somatic cell count; 2) health: blood serum chemistry, hematology, coagulation profile, and rumen fluid ammonia levels; 3) immune function: total serum immunoglobulins (IgA, IgG, IgM), specific antibody response to ovalbumin, and neutrophil phagocytosis. Dry matter intake, body weight, milk production, and milk composition were not affected by diet. Neutrophil phagocytosis was depressed throughout the experiment in cows fed the contaminated diet. Serum sodium concentrations and osmolality were significantly elevated throughout the experiment in cows fed the contaminated diet. Primary antibody response to ovalbumin immunization was higher in cows fed the contaminated diet compared with controls. It was concluded that feed naturally contaminated with Fusarium mycotoxins can affect metabolic parameters and immune function of dairy cows.