Protein degradation in Escherichia coli is accomplished by a handful of large oligomeric complexes. In most cases, these proteolytic machines are comprised of a chaperone (e.g. ClpA) that is required to prepare the substrate for degradation by the peptidase (e.g. ClpP). Recently, it was shown that the substrate recognition of the chaperone ClpA could be modified by the adaptor protein ClpS. To investigate the structural implications of this change in substrate specificity, ClpS was crystallized alone and in complex with the N-terminal domain of ClpA (ClpA(N)). Crystals of ClpS diffract to 2.9 A resolution and belong to space group P2(1)2(1)2(1), with unit-cell parameters a = 82.63, b = 145.67, c = 152.31 A. Two different crystal forms of the ClpA(N)-ClpS complex were characterized. Crystal form I (CFI) belongs to the orthorhombic space group P2(1)2(1)2, with unit-cell parameters a = 91.63, b = 112.47, c = 38.47 A; data to 1.92 A resolution were collected. Crystals of form II (CFII) belong to space group P4(1/3)2(1)2, with unit-cell parameters a = b = 93.57, c = 78.77 A, and diffract to 1.85 A resolution. Data sets collected from heavy-atom derivatives of CFI indicated the incorporation of Pt and Hg atoms. Structure solution using MIR and MAD methods is currently under way.