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The structure and physical properties of the fibrinolytic protein streptokinase have been investigated by 1H-n.m.r. spectroscopy. Well-resolved one- and two-dimensional spectra have been obtained for this molecule of molecular mass 47 kDa. Titration of all nine histidine residues has shown that these display a range of pKa values, between 5.6 and 8.2, revealing a variety of environments for these residues in the protein structure. Although at least eight histidine residues can be reversibly modified by diethylpyrocarbonate, only one is sufficiently exposed to be reactive towards photo-excited dye in chemically induced dynamical nuclear polarization spectroscopy experiments. Unfolding studies have been performed by thermal and chemical means. Evidence is presented here for several distinct unfolding transitions suggesting that the protein consists of at least three domains which have independent stability, and that the protein can exist in a number of partially folded states. For one of these, that formed in 2 M guanidine hydrochloride, it has been shown that the N-terminal region of the molecule is extensively unfolded, while other regions of the protein remain in native-like folded states.