The guanidine hydrochloride induced unfolding of the major fraction of ovalbumin (i.e. A1 which contains two phosphate groups and constitutes about 77% of the total protein) was investigated systematically by difference spectran and viscosity measurements. As judged by the intrinsic viscosity (3.9 ml/g), the native protein conformation is compact and globular. Difference spectral results showed extensive disruption of the native structure by guanidine hydrochloride with and without 0.1 M beta-mercaptoethanol were 31.1 and 27.0 ml/g. These and optical rotation results indicated that the denatured protein existed in a cross-linked random coil conformation in 6 M guanidine hydrochloride alone. Strikingly, in contrast to whole ovalbumin, the denaturation of its A1 fraction by guanidine hydrochloride was fully reversible and obeyed first-order kinetic law under different experimental condit ions of pH, temperature, and the denaturant concentration. The monotonic variation of deltaH for the unfolding of ovalbumin A1 by guanidine hydrochloride with temperature, the coincidence of the two transition curves obtained by measuring two independent properties (namely reduced viscosity and difference in light absorption at 288 nm (or 293 nm) as a function of the denaturant concentration, and finally the adherence of the unfolding as well as refolding reactions to first-order kinetic law suggested that the transition of ovalbumin. A1 can reasonably be approximated by a two-state mode. Analysis of the equilibrium data obtained at pH 7.0 and 25 degrees C according to Aune and Tanford (Aune, K.C.,and Tanford, C. (1969), Biochemistry 8, 4586) showed that 12 additional binding sites for the denaturant with an association constant of 1.12 were freshly exposed by the unfolding process and that the native protein was marginally more stable (approximately 6 kcal/mol) than its unfolded form even under native condition. The temperature dependence of the equilibrium constant for the unfolding of ovalbumin A1 by guanidine hydrochloride which was studied in the range 10-60 degrees C at pH 7.0 can be described by assigning the following values of the thermodynamic parameters for the unfolding process: deltaH = 52 kcal/mol at 25 degrees C; deltaS = 153 cal deg-1 mol-1 at 25 degrees C; and delta Cp = 2700 +/- 400 cal deg-1 mol-1.