This study was undertaken to determine the cell adhesion molecule profile of CD4+, CD8hi and CD56+ lymphocytes, which are mobilised to and from the peripheral blood during and after prolonged aerobic exercise. Ten healthy males (21-35 years old) were tested on two occasions, separated by at least 14 days. On the first occasion, subjects were examined in a rested state but did not exercise. On the second occasion, the same subjects were examined at the same time of day before, during and after 2 h of exercise at 65% of peak oxygen consumption. Blood samples obtained at rest (t0), during (at 0.5, 1, 1.5 and 2 h, t0.5, t1, t1.5 and t2, respectively) and after (at 4 and 24 h, t4 and t24, respectively) exercise were analysed by two-colour flow cytometry for CD4+, CD8hi and CD56+ cell surface expression, and density of CD62L, CD49d and CD11a. At t2, circulating concentrations of CD56+, CD8hi and CD4+ lymphocytes had increased (P < 0.05) by 330%, 105% and 30%, respectively. The majority of CD4+, CD8hi and CD56+ lymphocytes mobilised to the blood at t2 were CD62L- and CD11ahi, although populations of CD4+ and CD56+ cells that expressed CD62L+ and CD11alo were also mobilised. Changes in subset concentrations at t0.5 were positively associated (r = 0.63; P < 0.01) with their corresponding mean surface density of CD11a at t0. Our findings suggest that the differential mobilisation of lymphocytes during prolonged aerobic exercise is linked to the surface expression of CD11a (i.e. lymphocyte-function-associated antigen-1). However, mechanisms unrelated to CD11a expression also appear to be involved.