The U.S. Environmental Protection Agency Asbestos-in-Schools Program was established in March, 1979 to provide information and technical assistance to the public for addressing problems presented by asbestos-containing insulation materials in school buildings. Because there were no existing standard procedures for the qualitative and quantitative analysis of asbestos in bulk materials, the Office of Pesticides and Toxic Substances, Washington, DC, and the Environmental Monitoring Systems Laboratory, Research Triangle Park, NC, jointly sponsored an effort to produce a practical and objective analytical protocol. Draft procedures were written for the analysis of bulk samples by polarized light microscopy (PLM) and X-Ray powder diffraction (XRD). Following review, the Tentative Method for the Determination of Asbestiform Minerals in Bulk Insulation Samples (March, 1980) was submitted to a performance testing program that involved multiple laboratory analysis of prepared samples with known asbestos content. This report presents the results of the testing study and provides observations and preliminary characterization of the utility and operational parameters of the Tentative Method.