Addition of plasma to the photosensitizing agent N-aspartyl chlorin e6 (NPe6) in Triton X-100 micelles results in a blue-shift of fluorescence emission, corresponding to a change in the average environmental dielectric constant from 8 to 32. This effect was not observed with anionic or cationic detergents or with several other sensitizers including a close analog, chlorin e6. The apolipoprotein fraction of high-density lipoproteins (HDL) was found responsible for this blue-shift. These results indicate a highly selective interaction between HDL apolipoproteins and NPe6 molecules. Addition of the HDL apolipoprotein to NPe6 incorporated into Triton micelles decreased the fraction of NPe6 molecules accessible to Cd(2+)-induced fluorescence quenching but resulted in an increase in the Stern-Volmer quenching constant (an index of quenching efficiency).