Because obtaining multiple blood samples from individuals involved in epidemiologic studies is difficult, conclusions must be drawn on the basis of one or two samples. In this study, the authors attempted to determine the variability of two plasma markers over time, namely IL-6 levels and crosslinked fibrin degradation products (D-Dimers), in 16 elderly community-dwelling individuals. The study group included both men and women and black and white subjects (four in each group). Eight blood samples were obtained from each subject over a period of 36 days. Blood was separated within 1 hour after collection and aliquots of plasma were stored at -70 degrees C until all samples were collected. All samples from an individual were analyzed at one time. IL-6 and D-Dimers were measured by commercially available ELISA kits. The variability in the levels of plasma IL-6 and D-Dimers was assessed by means of intra-class correlation coefficient (ICC). The estimates of the ICCs for one measurement of IL-6 and D-Dimers were .87 and .86, respectively. Reliability values of this magnitude indicate excellent reproducibility in the measurement. These values indicate that obtaining a single sample from a subject is fairly representative of that individual's IL-6 and D-Dimer levels over an extended period of time.