The secondary structure of bovine serum albumin (BSA) was first examined in the thermal denaturation up to 130 degrees C. The helicity (66%) of the protein decreased with rise of temperature. Half of the original helicity was lost at 80 degrees C, but the helicity of 16% was still maintained even at 130 degrees C. When the BSA solution was cooled down to 25 degrees C after heating at temperatures above 50 degrees C, the helicity was not completely recovered. The higher the thermal denaturation temperature was, the lower was the recovered helicity. On the other hand, upon the addition of sodium dodecyl sulfate (SDS), the secondary structure of BSA was partially protected against the thermal denaturation above 50 degrees C where the structural change became irreversible. A particular protective effect was observed below 85 degrees C upon the coexistence of SDS of extremely low concentrations. For example, the helicity was 34% at 80 degrees C in the absence of SDS, but it was maintained at 58% at the same temperature upon the coexistence of 0.75 mM SDS. Upon cooling down from 80 to 25 degrees C, the helicity of BSA was recovered to 62% in the presence of 0.75 niM SIDS. Such a protective effect of SDS was not observed above 95 degrees C, In the interaction with the surfactant, this protein structure appeared likely to have a critical temperature between 90 and 100 degrees C in addition to the critical temperature in the vicinity of 50 degrees C. This protective effect of SDS, characterized by the specific amphiphilic nature of this anionic surfactant, is considered to be attained by building cross-linking bridges between particular nonpolar residues and particular positively charged residues in the protein molecule.