The effect of cortisone acetate (CA) on the immune response of control and furazolidone (FZ)-fed turkey poults was investigated. CA, fed at a dose of 500 mg/kg of ration beginning at 1 week of age, decreased mortality but had little effect on the development of FZ-induced cardiomyopathy. When poults were 2 weeks of age, the in vitro stimulation of lymphocytes by phytohemagglutinin (PHA) and concanavalin A (Con A) was significantly depressed (P less than or equal to 0.001 and P less than or equal to 0.05, respectively) in cortisone-treated poults. The time of the peak response of lymphocytes from poults 2-5 weeks of age to in vitro stimulation by PHA was significantly delayed (P less than or equal to 0.01) and the magnitude of the response was significantly depressed (P less than or equal to 0.001) in cortisone-treated poults compared with control poults. Cortisone treatment had no effect on time of peak response to Con A stimulation but significantly depressed (P less than or equal to 0.05) the magnitude of the response. Poults receiving FZ administered by gastric tube showed a peak response to PHA stimulation significantly (P less than or equal to 0.05) earlier and significantly (P less than or equal to 0.05) greater than did control poults.