Reversible unfolding of ovomucoid by guanidine hydrochloride, as followed by viscosity and difference-spectral measurements at 25 degrees C, pH6, occurred in two distinct steps involving at least three major conformational states, namely the native, intermediate and completely denatured states, occurring respectively in 60mm-sodium phosphate buffer, 3.5m-guanidine hydrochloride and 6m-guanidine hydrochloride. The overall native conformation of ovomucoid, as indicated by its intrinsic viscosity (5.24ml/g) and gel-filtration behaviour, differs significantly from that of a typical globular protein. Exposures of tyrosine residues in native ovomucoid measured by difference spectroscopy following perturbation with glycerol, ethylene glycol and dimethyl sulphoxide were, respectively, 0.42, 0.56 and 0.57. Of the exposed phenolic groups only one titrated normally (pK(int.), 9.91, electrostatic-interaction factor, w, 0.04). Results on difference spectra, solvent perturbation, phenolic titration and intrinsic viscosity (7.4ml/g) taken together showed that, although ovomucoid in 3.5m-guanidine hydrochloride was significantly unfolded, it retained a degree of native structure, removable with 6m-guanidine hydrochloride. In the latter, all the six tyrosine residues were available for titration, and the intrinsic viscosity of ovomucoid increased to 9.4ml/g. Furthermore, the characteristic fine structures in circular-dichrosim spectra of ovomucoid, associated with the elements of native structure, were abolished in 6m-guanidine hydrochloride, suggesting that the completely denatured state is structureless and presumably behaves as a cross-linked random coil. The latter state has been shown by analysis of the results on guanidine hydrochloride-dependence of the transition, intermediateright harpoon over left harpoondenatured, to be less stable than the intermediate state under native conditions by about 46kJ/mol at 25 degrees C. Attempts have been made to interpret the above results in the light of available information on the amino acid sequence of ovomucoid.