The interaction between an anionic conjugated polyelectrolyte, poly[9,9-bis(3'-butyrate)fluoren-2,7-yl] sodium (BBS-PF), and bovine serum albumin was investigated by fluorescence spectroscopy. The emission of BBS-PF was effectively quenched by BSA with a quenching constant K (SV) of 3.1 x 10(7) L/mol when BSA was at nanomolar concentrations, but the emission increased when the concentration of BSA was at micromolar level. The excitation band of BBS-PF blue-shifted when the emission was quenched where the negatively charged BSA induced the aggregation of BBS-PF, yet the excitation band of BBS-PF red-shifted when the emission increased where the BSA acted as a surfactant and formed hydrophobic interaction with BBS-PF. BBS-PF could also quench BSA through energy transfer by resonance with a quenching constant K (SV) of 1.1 x 10(6) L/mol. The emission band changes of BSA reflected the conformation transitions of BSA from class II to class I and the binding of BBS-PF with BSA made the BSA more folded.