A water-soluble fragment of the bc1 complex from bovine heart mitochondria was isolated containing the intact Rieske [2Fe-2S] cluster. The fragment consists of the last 129 amino acid residues of the Rieske iron-sulfur protein and has a molecular mass of 14592 Da including two iron atoms. The absorption, visible CD, and EPR spectra of the fragment are indistinguishable from those of the membrane-bound iron-sulfur protein. The redox potential as determined by EPR-monitored redox titration was + 306 mV. The far-ultraviolet CD spectrum is indicative of a protein with little regular secondary structure, while significant alpha-helix content was detected in the membrane anchor of the complete iron-sulfur protein. The fragment could be crystallized using poly(ethylene glycol) 6000 as precipitant. Needle-shaped single crystals have been grown by the hanging-drop vapor diffusion technique. These crystals belong to the space group P21 and diffract well beyond 0.2 nm resolution. Phase determination using the multiple-wavelength anomalous-scattering technique is underway.