We measured the in vivo and in vitro effects of interferon (IFN)beta and glatiramer acetate (GA) on the expression of the regulatory molecule, tumor necrosis factor related apoptosis inducing ligand (TRAIL), in patients with multiple sclerosis (MS). We confirmed the prior observation that TRAIL is enhanced on anti-CD3 activated T cells by the in vitro addition of IFNbeta. T cells from IFNbeta-treated patients stimulated with anti-CD3 only, had higher levels of TRAIL than untreated patients, suggesting that in vivo IFNbeta exposure has an effect on TRAIL expression in association with T cell activation. In vitro IFNbeta-induced TRAIL upregulation on anti-CD3 or phytohemagglutinin-activated T cells was comparable for IFNbeta-treated and non-treated MS patients and controls, indicating that IFN receptors were neither saturated nor down-regulated by current IFNbeta therapy. Although GA in vivo or in vitro did not induce TRAIL, the IFNbeta +GA combination in vitro enhanced TRAIL expression to higher levels than IFNbeta alone on CD4+ T cells obtained from MS patients, regardless of GA treatment status, and healthy donors, and on GA reactive T cell lines derived from GA-treated patients or controls. Whether any observed therapeutic effects of GA/IFNbeta combination therapy will correlate with TRAIL expression and function remains to be determined.