Detected levels of IL-6, TNF-alpha and leptin may be affected by methods of storage, anticoagulant or repeated freezing-thawing. Blood samples from 22 healthy subjects were: (i) allowed to stand for 1, 2, 4 or 6 h prior to, or after separation, before freezing at -70 degrees C; (ii) taken into tubes with lithium heparin, sodium citrate, EDTA or no anticoagulant, separated and frozen; and (iii) separated, and plasma repeatedly freeze-thawed for up to six cycles prior to assay. Leptin was assayed by RIA, and IL-6 and TNF-alpha by high-sensitivity ELISA. (i) IL-6 and TNF-alpha levels were not altered significantly in separated samples but IL-6 declined by mean (SEM) 14.3% (3.7%) and TNF-alphaincreased by 9.6% (2.3%) in samples left unseparated for 4 h (P=0.003 and 0.002, respectively). Leptin remained unchanged. (ii) Serum and EDTA-plasma samples gave comparable results for all three cytokines, but levels in the other anticoagulant samples were highly variable. (iii) IL-6 and leptin levels were not altered by up to 6 cycles of freeze-thawing, but TNF-alpha increased by 17.0% (3.7%) after 3 cycles. Concentrations of these molecules are significantly altered by storage conditions, therefore they need to be standardized for epidemiological and clinical studies, and between-study comparisons of levels may not be reliable. Copyright 2000 Academic Press.