Interactions and characteristics of 0.1% bovine serum albumin (BSA)-sodium dodecyl sulfate (SDS) in 20 mM phosphate buffer solution were investigated by means of fluorescence spectroscopy and electron spin resonance (ESR) spectroscopy. In BSA-SDS system, the intensity ratio, Im3/Im1, of the third vibronic band of the pyrene monomer to the first vibronic band showed a small peak at about 0.1 mM SDS in the phosphate buffer below cmc. In accordance with this Im3/Im1 ratio, the intensity ratio, Ie/Im1, of fluorescence from the pyrene excimer to that from the monomer showed a pseudo-plateau (0.08-0.8 mM) and suggested the existence of micelle-like aggregates below the cmc. Temperature dependence of ln(Ie/Im1) in pyrene fluorescence in the SDS-BSA system was examined as a function of SDS concentration. The activation energy of pyrene diffusion for excimer formation in a micelle was estimated to be 19.2 kJ mol(-1) for the BSA-SDS system. ESR spectra of 5-doxylstearic acid (5-DSA) showed that the probe location is restricted at SDS concentrations above the cmc, and that the probe also is highly restricted in motion for BSA-bound SDS micelles.