The toxic equivalency factor (TEF) approach has been widely accepted as the most feasible method available at present for evaluating potential health risks associated with exposure to mixtures of dioxin-like compounds (DLCs). The current mammalian TEFs for the DLCs were established by the World Health Organization (WHO) following the meeting of an international expert panel in June of 1997. The TEFs recommended by WHO were determined based on a consensus of scientific judgment and were presented as point estimates. However, the relative potency estimates (REPs) underlying the TEFs were derived from a heterogeneous data set and often span several orders of magnitude. In this article, we present a refined database of mammalian REPs that we believe will facilitate better characterization of the variability and uncertainty inherent in the data. The initial step involved reviewing the REP database used by the WHO panel during its review in 1997. A set of criteria was developed to identify REPs that were determined to be the most representative measure of a biological response and of adequate quality for use in quantitative analyses. REPs were determined to be inappropriate for use in quantitative analyses if any of the established exclusion criteria were met. Comparison of data records to the established exclusion criteria resulted in the identification of a substantial number of REPs believed to be inappropriate for use in quantitative analyses. Next, studies published after 1997 were added to the database. The availability of such a refined database will improve risk assessment for this class of compounds by including additional information from new studies and facilitating the use of quantitative approaches in the further development of TEFs.