The effects of rhG-CSF on peripheral blood lymphocytes and lymphocyte populations in the apheresis product has been determined in 13 individuals (11 autografts and 2 normal donors) who had peripheral blood mononuclear cells (PBMCs) collected on days 3, 4, and 5 of administration of rhG-CSF 16 micrograms/kg/day x 5 days. The absolute number of CD34+ cells increased 9 and 25-fold from pretreatment levels after 4 and 5 days of rhG-CSF, respectively. All patients demonstrated an increase in CD3, CD4, CD8, CD19 and CD20 lymphocytes after 3 days of rhG-CSF with T lymphocytes increasing 1.5-2.0 times baseline by day 3 or rhG-CSF administration. All lymphocyte phenotypes returned to below pretreatment levels on days 4 and 5 of rhG-CSF administration. The ratio of CD4/CD8 lymphocytes was not affected by rhG-CSF. Collection of PBMCs on 3 consecutive days yielded a mean of 8.77 x 10(8) CD34 cells, 14.03 x 10(10) total nucleated cells and 3.17 x 10(10) CD3 lymphocytes. These data suggest that rhG-CSF mobilized PBMCs have approximately one log more T cells than marrow and the effect of rhG-CSF on the quantity and phenotype of lymphocytes is minimal. Strategies for coping with an increased incidence of GVHD, if it occurs, could include the utilization of both methotrexate and cyclosporine as immunoprophylaxis, selective T cell depletion or CD34 positive selection.