48 healthy donors underwent peripheral blood stem cell (PBSC) apheresis for allogeneic transplantation beginning on day 4 of G-CSF (2 x 5 microg/kg) mobilization. In one to four (median two) large-volume mononuclear cell aphereses, a median of 55.9 x 10(9) of lymphocytes (range 21.0-109.2 x 10[9]) were collected, an amount comparable to lymphocyte numbers removed by therapeutic lymphaphereses in autoimmune diseases. Mean peripheral lymphocyte counts decreased from premobilization values of 2.31 x 10(9)/l to 1.31 x 10(9)/l at a median of 34 d (1 month) and 1.53 x 10(9)/l at a median of 327 d (11 months). The decrease in peripheral lymphocyte counts was significantly correlated with the number of lymphocytes removed and the number of aphereses. Neutrophil and platelet counts returned to normal values after 1 month whereas monocyte counts and haemoglobin concentrations were significantly decreased at 1 month but not at 11 months.