The effect of granulocyte colony-stimulating factor (G-CSF) on peripheral blood lymphocytes (PBL) and CD34+ cell frequency in the apheresis product has been determined in 25 healthy stem cell donors. Peripheral blood mononuclear cells (PBMNC) were collected after five days of G-CSF 10 microg/kg/day s.c., which was well tolerated. The median number of leukocytes increased eight-fold over that of pretreatment levels. Collection of PBMNC lasted a median of two (range, 1-3) days. The mean mononuclear cell (MNC) count and total lymphocyte percentage were 6.69 x 10(8)/kg and 59.08%, respectively, and the frequency of CD34+ cell expression was 2.1% in the apheresis product. The frequency of CD3+, CD4+, CD25+, NK and CD122+ cell expressions in mobilized PBMNC and PBL showed no significant difference. However, the frequency of CD8+, CD8+28+, CD3+DR+, CD19+, CD20+ and CD22+ B cells expression in the apheresis product increased significantly compared to steady-state PBL. In contrast, the frequency of the CD11 a+ and CD8+38+ cell expressions in the apheresis product was decreased compared to the steady-state PBL. The mean yield of CD34+ and CD3+ cells were 13.6 x 10(6) and 2.69 x 10(8)/kg of recipient body weight (RBW), respectively. Following allograft all patients engrafted with >0.5 x 10(9)/l neutrophil and < or = 20 x 10(9)/l platelets on a median of day 13 and 12, respectively. Nine patients had grade II-IV acute GVHD and chronic GVHD occurred in eight patients. Four patients died due to transplant-related complications. There was one late engraftment failure which occurred on the fifth month. Thirteen patients are still alive. In conclusion, these results indicate that administration of G-CSF at 10 microg/kg/day in normal donors alters the lymphocyte subsets and there are significant differences in the lymphocyte contents of the recipients before apheresis and in apheresis product.