Diets containing 200 mg fumonisin B1/kg of feed and .75 mg aflatoxin/kg of feed singly or in combination were fed to female turkey poults (Nicholas Large White) from day of hatch to 21 d of age. When compared with controls, 21-d body weight gains were reduced 10% by fumonisin B1, 39% by aflatoxins, and 47% by the combination. Relative weights (grams/100 g body weight) of the kidney and pancreas increased in poults fed the diet containing aflatoxins alone, whereas the relative weight of the liver decreased. Relative weights of the liver and pancreas increased in the poults fed the fumonisin diet. Relative weights of the kidney, pancreas, and gizzard increased in the poults fed the combination diet, whereas the relative weight of the liver decreased. Most serum constituents, hematology values, and activities of enzymes measured were altered in poults receiving the diets containing aflatoxins with or without fumonisin B1. No major histological lesions were observed in tissues from control poults or poults fed the diet containing fumonisin alone. Lesions associated with aflatoxins were only observed in the liver and occasionally in the kidney of poults fed the diets containing aflatoxins with or without fumonisin B1. The primary hepatic change was bile duct hyperplasia with some hepatocellular degeneration and necrosis and megalocytosis. Occasional necrotic and degenerating tubular epithelial cells were observed in the kidneys. The increased toxicity in poults fed the combination diet for most variables can best be described as additive, although some variables showed less than additive toxicity.(ABSTRACT TRUNCATED AT 250 WORDS)