The structure of streptokinase in solution has been studied by dynamic light scattering, small-angle X-ray scattering and circular dichroism spectroscopy. The Stokes' radius and radius of gyration of the protein monomer are 3.58 nm and 4.03 nm, respectively. The maximum intraparticle distance of the molecule is 14 nm. More than half of the amino acids of the molecule are organized in regular secondary structures. The X-ray scattering curve, the results from dynamic light scattering, and the finding that at least 50% of the amino acid residues are organized in regularly folded secondary structures are consistent with the following structural model. Streptokinase consists of four compact, separately folded, domains linked by mobile segments of the protein chain. The molecule exhibits the conformation of a flexible string-of-beads in solution.