Background: Interest in biomarker patterns and disease has led to the development of immunoassays that evaluate multiple analytes in parallel while using little sample. However, there are no current standards for multiplex configuration, validation, and quality. Thus, validation by platform, population, and question of interest is recommended. We sought to determine the best blood fraction for multiplex evaluation of circulating biomarkers in post-menopausal women, and to explore body composition phenotype discrimination by biomarkers. Methods: Archived serum and plasma samples from a sample of healthy post-menopausal women with the highest (n=9) and lowest (n=11) percent lean mass, as determined by dual-energy X-ray absorptiometry, were used to measure 90 analytes using bead-based, suspension multiplex assays. Replicates of serum and plasma were analyzed in a random selection of four of these individuals. Results: Ninety percent of the analytes were detectable for ≥50% of samples; when limited to these well detected analytes, mean replicate correlations for serum and plasma were 0.87 and 0.85, respectively. Serum had lower error rates discriminating phenotypes; seven serum vs. two plasma analytes discriminated extreme body phenotypes. Conclusions: Serum and plasma performed similarly for the majority of the analytes. Serum showed a slight advantage in predicting extreme body composition phenotypes in postmenopausal women using parallel evaluation of analytes. © 2011 by Walter de Gruyter Berlin New York.