This report documents progress made on the subject project during the period of September 1, 2003 through February 28, 2004. The TERESA Study is designed to investigate the role played by specific emissions sources and components in the induction of adverse health effects by examining the relative toxicity of coal combustion and mobile source (gasoline and/or diesel engine) emissions and their oxidative products. The study involves on-site sampling, dilution, and aging of coal combustion emissions at three coal-fired power plants, as well as mobile source emissions, followed by animal exposures incorporating a number of toxicological endpoints. The DOE-EPRI Cooperative Agreement (henceforth referred to as ''the Agreement'') for which this technical progress report has been prepared covers the analysis and interpretation of the field data collected at the first power plant (located in the Upper Midwest), followed by the performance and analysis of similar field experiments at two additional coal-fired power plants utilizing different coal types and with different plant configurations. Modifications to the original study design, which will improve the atmospheric aging component of the project and ensure that emissions are as realistic as possible, have resulted in project delays, and, at the time of report preparation, fieldwork at the Upper Midwest plant had not begun. However, such activities are imminent. This report therefore does not present data for activities covered by the Agreement, but does present results for the laboratory methods development work. This work is critical for the future success of the project. In particular, the atmospheric reaction simulation system is of paramount importance to the TERESA study design, since the basis for the toxicity assessment lies in the generation of realistic exposure atmospheres. The formation, composition, and toxicity of particles will be related to different atmospheric conditions and plume dilution scenarios through variations in reaction conditions. Because of the critical role played by this component in ensuring the overall success of the project, more time was required to develop and optimize the system, and the one-chamber simulation system outlined in the original Scope of Work for the Agreement was modified to comprise a more realistic dual chamber system. We are confident that the additional time required to optimize these methodologies will result in a significant improvement in the study. We fully expect that results for tasks covered under the Agreement, and a complete discussion of their relevance and value, will be included in the next semiannual progress report.