Young female broiler chickens fed diets amended with 0, 35, 75, and 150 mg fusaric acid (FA)/kg diet for 3 weeks showed no aberrations in behavior, feed intake, weight gain, or appearance of the visceral organs. Furthermore, there was no correlation between the dietary concentration of FA and incidence of tibial dyschondroplasia and leg-shape deformities. Ash content of dry fat-free tibiae was not influenced by FA; thus, no rickets was present in these chickens. FA enhanced the humoral response to sheep erythrocytes but significantly reduced cell-mediated cutaneous response to phytohemagglutinin-P.