Changes in membrane polarity of human platelets during the interaction with PAF were investigated by measuring the steady-state fluorescence emission spectra of 2-dimethylamino(6-lauroyl)naphthalene (Laurdan), which is known to be incorporated at the hydrophobic-hydrophilic interface of the bilayer, displaying spectral sensitivity to the polarity of its surroundings. Laurdan shows a marked steady-state emission red-shift in polar solvents, with respect to non-polar solvents. Our results demonstrate that platelet activation factor (PAF) (10(-7) M) induces a red-shift of the fluorescence emission spectra of Laurdan. These changes were not observed in the presence of the PAF antagonist, L-659,989. These data suggest that the interaction of PAF with its specific receptor and the signalling pathways involved in platelet activation are accompanied by an increase in polarity at the hydrophobic-hydrophilic interface of human platelet membranes.