IL-4 and IFNγ play central roles in the development and maintenance of immune responses on mucosal surfaces. The goal of our studies was to gain insight into the mechanisms by which IL-4 and IFNγ regulate ion transport and barrier functions of airway and intestinal epithelial cells. Cystic fibrosis transmembrane conductance regulator (CFTR) protein expression is inhibited by both IFNγ (Ki=0.54ng/ml) and by IL-4 (Ki=0.195 ng/ml) in HT29-CL19A colonic epithelial cells and in Calu-3 airway epithelial cells. Reduced CFTR protein expression is paralleled by a decrease in cAMP-elicited Cl- secretion as assayed by short circuit current (Isc). Unlike their similar effects on CFTR expression and Cl- secretion, IL-4 and IFNγ have opposing effects on the regulation of transepithelial resistance (Rte). IL-4 decreases, while IFNγ increases Rte. Both cytokines act only from the basolateral side. Sphingosine (100μM), an inhibitor of protein kinase C (PKC), blocked the inhibitory effect of IL-4 on CFTR protein expression. Sphingosine did not change the inhibition of CFTR expression by IFNγ or the regulation of Rte by either IL-4 or IFNγ. These data suggest that the IL-4 induced decrease in CFTR protein expression is mediated by PKC. Identifying regulatory mechanisms of CFTR expression might aid the design of novel CF therapies that are based on the stimulation of expression and processing of mutant CFTR.