Metabolites of polychlorinated biphenyls (PCBs) bind with high affinity to uteroglobin, a small homodimeric protein that also binds progesterone. We present the solution structure of the reduced form of rat uteroglobin in complex with a PCB methylsulphone, (MeSO2)2-TCB. The structure reveals the molecular basis for the accumulation of (MeSO2)2-TCB by uteroglobin. The structure also shows how ligand binding and release might be controlled by reduction/oxidation of two intermolecular disulphide bonds. Breakage of these bonds induces a local unfolding of the N- and C-termini and a separation of helices creating a channel into the binding site. These effects make the ligand binding cavity readily accessible to entry of the ligand.