Studies suggest that physical fitness promotes cardiovascular health, including improved endothelial function and possibly reduced inflammatory responses to stressors. This study examined the effects of fitness on leukocyte - endothelial adhesion in response to an acute exercise challenge. Peripheral blood mononuclear cell (PBMC) adhesion to human umbilical venous endothelial cells (HUVEC) was examined in 18 fit and 19 non-fit individuals (mean age 39 years, SD ± 11) before and after a 20-minute treadmill exercise at 65-70% VO_2peak oxygen consumption. PBMC were isolated from whole blood (Ficoll-Paque) at rest and immediately following exercise. HUVEC were incubated for 4 hours in the presence of cytokines IL-1 & IL-8 in order to activate endothelial intercellular adhesion molecule (ICAM-1) expression. Fit subjects showed a more than 2-fold reduction in PBMC - HUVEC adhesion after exercise (p<0.01) as compared to less fit subjects, who showed no significant change. Regardless of fitness levels, both at rest and in response to exercise, soluble ICAM-1 in the incubation media attenuated PBMC - HUVEC adhesion approximately 81% (p<0.001). The findings indicate that immune cells that demarginate in response to exercise have reduced ability to adhere in individuals who are physically fit, an effect apparently independent of ICAM-1 binding. The findings provide evidence of how physical fitness might protect individuals from excessive inflammatory responses to exercise.