Using monoclonal antibodies as probes, we have characterized three antigens with respect to localization in the nucleolus, molecular weight and solubility. Two proteins, of 110,000 and 94,000 apparent molecular weight, were found associated with the ribonucleoprotein fibers. A third protein, with a molecular weight of 40,000, was accumulated at the nucleolar periphery, was present in the nucleoplasm, and may be involved in pre-ribosome maturation and transport.