The effects of feeding Fusarium moniliforme culture material, containing known concentrations of fumonisin B1 (FB1), were studied in broiler chicks. Day-old chicks were allotted randomly to dietary treatments containing 0, 1.02, 2.04, 3.06, 4.08, 5.10, 6.12, and 7.14% fumonisin culture material (FCM). These levels of FCM supplied 0, 75, 150, 225, 300, 375, 450, and 525 mg of FB1/kg of feed. Each dietary treatment was fed to four pen replicates of six birds each for 21 days. Chicks fed FCM that supplied 450 and 525 mg FB1/kg diet had lower (P < .05) feed intakes and BW gains; increased (P < .05) liver and kidney weights; and increased (P < .05) mean cell hemoglobin, and mean cell hemoglobin concentrations. Compared with controls, chicks fed FCM had increased (P < .05) free sphinganine levels and sphinganine:sphingosine ratios. Treatment-associated histological lesions were only observed in the liver of chicks fed diets containing FCM that supplied 225 mg FB1/kg or higher. Diets containing FCM that supplied levels as low as 75 mg FB1/kg affected the physiology of chicks by increasing free sphinganine levels and sphinganine:sphingosine ratios. Because inhibition of sphingolipid biosynthesis has been hypothesized as the mechanism of action of FB1, this suggests that diets containing 75 mg FB1/kg FCM may be toxic to young broiler chicks.