The spin-labeling method was used to investigate human carbonic anhydrase, HCA II, undergoing unfolding induced by guanidine-HCI (Gu-HCI). The spin-probe, N-(2,2,5,5-tetramethyl-1-yloxypyrrolidinyl-3-yl)iodoacetamide, was attached covalently to the single cysteine (position 206) in the enzyme. The electron paramagnetic resonance spectrum of the folded structure showed the characteristic slow motional spectra. When the concentration of the denaturing agent, Gu-HCI, was gradually increased, new spectral components with narrower lines evolved to give complex electron paramagnetic resonance spectra, apparently containing superimposed contributions from several components of different mobility. By a differentiation technique, it was possible to follow the relative increase of the narrow components as a function of Gu-HCI concentration. The amplitude of difference spectra versus Gu-HCI concentration showed two distinct maxima, indicating the existence of a folding intermediate state/structure. The results were found to agree with optical absorption data, which showed similar transitions at the same Gu-HCI concentrations. From line-shape simulations assuming a Brownian diffusion model, the rotational diffusion constants for the spin-label in the folded, folding intermediate, and unfolded structures were determined. The relative abundances of the three conformations in the region 0-4 M Gu-HCI were obtained by least squares fitting of the simulated spectra to the experimental ones. The folding intermediate was found to have a maximum population of 39 +/- 4% at approximately 0.7 M Gu-HCI.