1. The nonsteroidal drug ibuprofen exists as an R(-)- and S(+)-enantiomer. Only the S(+)-enantiomer is an effective cyclo-oxygenase inhibitor, while the R(-)-enantiomer is inactive in this respect. Thus the molecular mechanism by which R(-)-ibuprofen exerts its anti-inflammatory and antinociceptive effects remains unknown. 2. In this study the effects of the enantiomers of ibuprofen on modulation of transcription factors have been examined with electrophoretic mobility-shift assay (EMSA), transient transfection experiments, confocal immunofluorescence and nuclear import experiments, to determine their selectivity and potency as inhibitors of the activation of transcription factor nuclear factor-kappaB (NF-kappaB). 3. R(-)-ibuprofen (IC50: 121.8 microM) as well as the S(+)-enantiomer (IC50: 61.7 microM) inhibited the activation of NF-kappaB in response to T-cell stimulation. The effect of ibuprofen was specific because, at concentrations up to 10 mM, ibuprofen did not affect the heat shock transcription factor (HSF) and the activation of NF-kappaB by prostaglandin E2 (PGE2). Very high concentrations of ibuprofen (20 mM) did not prevent NF-kappaB binding to DNA in vitro. Immunofluorescence and nuclear import experiments indicate that the site of ibuprofen action appeared to be upstream of the dissociation of the NF-kappaB-IkappaB-complex. 4. Our data raise the possibility that R(-)-ibuprofen exerts some of its effects by inhibition of NF-kappaB activation.