The αLβ2 (CD1la/CD18) integrin LFA-1 (lymphocyte function-associated antigen-1) mainly contributes to the firm arrest of leucocytes on the endothelium. Its cell surface density is thought to be important for the exercise-induced homing of leucocytes 9 athletes (7 males, 2 females, age 36-68 years, body mass 64 ± 10kg, height: 175 ± 10cm) absolved a competitive 100km run in 8:07 hours (range: 7: 29-9: 50 hours). Immunophenotyping of circulating leucocytes and data aquisition by three colour flow cytometry before and up to 3 hours after the race showed that post exercise lymphocyte subpopulations with a higher expression of CD11a like CD3+CD8+CD45RO+ T-cells, natural killer cells (CD3-CD16/CD56+) and cytotoxic, not MHC-restricted T-cells (CD3-CD16/CD56+) decreased significantly more than CD45RO+ and CD45RO- helper-T-cells (CD3+CD4+), CD3+CD8+CD45RO- T-cells and B-cells (CD 19+), which have lower levels for CD11a. Cell concentrations of regular monocytes (Fey receptor 3 negative, LFA-1 low positive) increased, whereas mature monocytes (Fey receptor 3 low or high positive respectively, LFA-1 high positive, decreased. In conclusion the surface density of the adhesion molecule LFA-1 on leucocytes is likely to contribute significantly to the extent of leucocyte's homing after long endurance exercise.