A binding protein for certain methyl sulfone metabolites of polychlorinated biphenyls (PCB) was partially purified from lung cytosol of untreated female rats. The protein has an Mr of 13,000 and a pI of 5.3 in the absence of reducing agents. In the presence of dithioerythreitol or beta-mercaptoethanol, the protein is split into subunits with a more basic pI. The 13-kDa protein was electroeluted from SDS-polyacrylamide gels, and an antiserum against the protein was raised in rabbit. The immunoglobulin fraction was shown to contain monospecific antibodies against the 13-kDa protein as determined by Western immunoblots. The antibodies retained partially purified binding protein labeled with radioactive ligand when subjected to protein A-Sepharose chromatography and caused a shift in the elution of the labeled protein from Sephadex G-75 and a shift in its sedimentation behavior on sucrose gradients. Due to striking similarities in physicochemical characteristics of the 13-kDa protein and a protein purified from rabbit lung and uterus, uteroglobin, the anti 13-kDa protein antibodies were tested for cross-reactivity. As judged by Western immunoblots, the anti 13-kDa protein antibodies did not cross-react with uteroglobin and the two proteins, although similar, do not seem to be identical. The 13-kDa protein is proposed to be responsible for the accumulation of certain methylsulfonyl-PCBs in lung tissue of rats. Monospecific antibodies against the 13-kDa protein should constitute immunochemical probes of great value in attempts to elucidate the physiological role of the protein as well as its possible role in PCB-induced respiratory toxicity.