PCBs and other organochlorines find a stable reservoir in fat; however, they are excreted to the bloodstream with lipid mobilization during times of negative energy balance, such as weight loss. This process of organochlorine mobilization exposes cells and tissues to increased concentrations of PCBs, thereby creating a window for enhanced toxicologic effects. Aims of this study are to investigate associations between exercise-induced weight loss, PCB concentrations, and immune function among postmenopausal women in Seattle, WA. The study builds on resources from an existing randomized controlled trial of exercise, with an extensive assessment of immune function biomarkers. We will measure PCB concentrations in plasma samples from 115 participants, both prior to the intervention and at a 12-month followup visit. Data on PCB concentrations and existing trial data will be used to: 1) Evaluate relationships between energy balance characteristics and plasma PCBs, both prior to the intervention and across the trial period; and 2) Investigate immune effects of PCB exposure, both prior to the intervention and following any changes in PCB concentrations that result from the exercise intervention. We will explore how circulating PCB concentrations are associated with specific aspects of energy balance such as weight and weight cycling history, body composition including abdominal adiposity, and changes in weight and body composition during the intervention. Additionally, the study design will allow investigation of how PCB 'dose' resulting from weight loss during the trial affects immune function measures such as lymphocyte phenotypes, natural killer cell cytotoxicity, and lymphocyte proliferation.