Upon the addition of small amounts of sodium dodecyl sulfate (SDS), the helicity of bovine serum albumin (BSA), lost in the urea denaturation, was mostly recovered. The profile of the recovery differed depending on the urea concentration. The urea concentrations were then divided into three ranges. (1) A range below (3 M) where the helicity only decreased, as in the absence of urea (the helicity decreased down to (47 %) in the SDS solution). (2) A range between 4 and (8 \mathrm{M}) where the helicity initially increased up to (62 %) and then sharply decreased below (2 \mathrm{mM}) SDS. (3) A range above (9 M) where the helicity only increased with an increase in SDS concentration. In each case, the helicity finally reached a value of around (47 %), attained in the SDS solution without urea. In the middle concentration range, profiles of the structural change were rather complicated. It is worth noting that the helicity is recovered upon the addition of SDS of less than (0.3 \mathrm{~m} M) in this range except for in the (8 M) urea case. At (8 M) urea, the helicity is recovered upon the addition of (1.0 \mathrm{~m} M) SDS. These results were obtained upon the addition of SDS to BSA denatured by urea. However, the same helicity was obtained at each SDS concentration when the surfactant was added prior to the urea denaturation. It appears likely that the SDS denaturation finally predominates over the urea denaturation. The same competition was observed in the absorbance change in the aronatic region, "3 1993, Acadenic liress, Inc.