Mechanical loading of cells induces the expression of transforming growth factor-beta-1, and acute exercise, which involves mechanical loading of several tissues, could thus increase its circulating level in humans. However, no consensus exists regarding the plasma concentration of this cytokine in resting subjects (reported values range from 500 to 18,300 pg ml(-1)) and also the extent of intra-individual variation is unknown. As a basis for detecting exercise-induced changes in transforming growth factor-beta-1, we measured its concentration, by enzyme-linked immunosorbent assay, in plasma from eight healthy resting subjects. Plasma was sampled from each subject on five successive days according to a procedure designed to minimize activation of platelets, as platelet alpha-granules contain large amounts of transforming growth factor-beta-1. The mean plasma level was relatively low [1155 (30) pg ml(-1), mean (SE)], and did not differ between days, indicating that platelet activation was minimal. Several alterations in the blood sampling procedure did not affect results, while a 40% increase was seen when blood was not cooled appropriately prior to centrifugation. A moderate intra-individual variation (average CV=9.8%) indicated a stable plasma level at rest. In response to exercise (1 h of treadmill running) the plasma concentration of transforming growth factor-beta-1 increased from 992 (49) pg ml(-1) (at rest) to 1301 (39) pg ml(-1) (post exercise) ( P