To examine the toxic effects of fumonisin B1 (FB1(-containing culture material and deoxynivalenol (DON)-contaminated wheat diets on barrows. 24, 7-week-old crossbred barrows allotted to 4 equal groups of 3 replicates of 2 barrows/replicate. Barrows were fed diets for 28 days that were formulated as follows: no additional FCM or DON/kg of feed (control); 100 mg FB1/kg of feed; 5 mg DON/kg of feed; or 100 mg FB1 plus 5 mg DON/kg of feed. Body weight and feed consumption were monitored weekly. On day 28, blood samples were obtained for serum biochemical, hematologic, and immunologic measurements. On day 29, barrows were euthanatized and necropsies were performed. Analyzed mycotoxin content of diets were: none detected (control); 47 mg of FB1/kg of feed (FB1 diet); 4.5 mg of DON/kg of feed (DON diet); and 56 mg of FB1 and 3.7 mg of DON/kg of feed (FB1 plus DON diet). Differences were detected among groups of barrows for clinical performance, serum biochemical analytes, immunologic response, and histopathologic lesions. Combining FB1-containing material and DON-contaminated wheat in the diets of growing barrows induces a more toxic response than that induced by either toxin singly. For many variables, the response could be described as additive; however, for some variables, responses were interactive in a greater-than-additive manner. Caution should be exercised when formulating swine diets that could contain FB1 and DON, because the condition induced by their combination is more severe than that predicted for each mycotoxin's toxicity. (Am J Vet Res 1996;57:1790–1794) To examine the toxic effects of fumonisin B1 (FB1(-containing culture material and deoxynivalenol (DON)-contaminated wheat diets on barrows. 24, 7-week-old crossbred barrows allotted to 4 equal groups of 3 replicates of 2 barrows/replicate. Barrows were fed diets for 28 days that were formulated as follows: no additional FCM or DON/kg of feed (control); 100 mg FB1/kg of feed; 5 mg DON/kg of feed; or 100 mg FB1 plus 5 mg DON/kg of feed. Body weight and feed consumption were monitored weekly. On day 28, blood samples were obtained for serum biochemical, hematologic, and immunologic measurements. On day 29, barrows were euthanatized and necropsies were performed. Analyzed mycotoxin content of diets were: none detected (control); 47 mg of FB1/kg of feed (FB1 diet); 4.5 mg of DON/kg of feed (DON diet); and 56 mg of FB1 and 3.7 mg of DON/kg of feed (FB1 plus DON diet). Differences were detected among groups of barrows for clinical performance, serum biochemical analytes, immunologic response, and histopathologic lesions. Combining FB1-containing material and DON-contaminated wheat in the diets of growing barrows induces a more toxic response than that induced by either toxin singly. For many variables, the response could be described as additive; however, for some variables, responses were interactive in a greater-than-additive manner. Caution should be exercised when formulating swine diets that could contain FB1 and DON, because the condition induced by their combination is more severe than that predicted for each mycotoxin's toxicity. (Am J Vet Res 1996;57:1790–1794)