Exercise affects t-cell cytokine production. Whether or not these effects are caused by circulating factors associated with physical activity (e.g., inflammatory mediators, acidosis) is unknown. To investigate this, we incubated sera (10%), obtained from 16 young-adults before (PRE) and after (END) 30-min of exercise, with commercially available Jurkat cells, a t-lymphocyte model, that, of course, had never been exposed to an exercise milieu. After 1 and 6h in culture, we measured in the supernatant four cytokines (each known to be altered by exercise and involved in disease pathophysiology): IL-2, TGF-beta1, TNF-alpha, and IL-1ra. Cell proliferation was assessed with proliferating nuclear cell antigen (PNCA). Statistical analysis consisted of a linear mixed model for repeated measurement. There was no effect of exercise on t-cell production of either TGF-beta1 or IL-1ra. In contrast, both IL-2 (p=0.025) and TNF-alpha (p=0.031) production was significantly suppressed in sera from the exercising participants. The suppression of these two cytokines occurred despite the fact that PNCA significantly increased (p=0.0004) in the END serum. In conclusion, exercise alters circulating factors that can, subsequently, influence t-cell cytokine production in vitro.