PURPOSE The optimal dose of granulocyte colony-stimulating factor (G-CSF) for mobilization of allogeneic-blood stem cells (AlloBSC) has yet to be determined. As part of a prospective trial, 41 related human leukocyte antigen (HLA)-matched donors had blood cells mobilized with G-CSF at 5 micrograms/kg/d by subcutaneous administration. The purpose of this trial was to monitor adverse effects during G-CSF administration and stem-cell collection, to determine the optimal timing for stem-cell collection, and to determine the cellular composition of stem-cell products following G-CSF administration. PATIENTS AND METHODS The median donor age was 42 years. Apheresis began on day 4 of G-CSF administration. At least three daily 12-L apheresis collections were performed on each donor. A minimum of 1.0 x 10(6) CD34+ cells/kg (recipient weight) and 8.0 x 10(8) mononuclear cells/kg were collected from each donor. All collections were cryopreserved in 5% dimethyl sulfoxide and 6% hydroxyethyl starch. RESULTS Toxicities associated with G-CSF administration and the apheresis process included myalgias/arthralgias (83%), headache (44%), fever (27%), and chills (22%). The median baseline platelet count of 242 x 10(4)/ mL decreased to 221, 155, and 119 x 10(6)/mL on days 4, 5, and 6 of G-CSF administration, respectively. Median numbers of CD34+ cells in collections 1, 2, and 3 were 1.99, 2.52, and 3.13 x 10(6)/kg, respectively. The percentage and total number of CD4+, CD8+, and CD56+/CD3- cells remained relatively constant during the three collections. Median total numbers of cells were as follows: CD34+, 7.73 x 10(6)/kg; and lymphocytes, 6.93 x 10(8)/kg. CONCLUSION Relatively low doses of G-CSF can mobilize sufficient numbers of AlloBSC safely and efficiently.