Peripheral blood stem cells are widely used as stem cell source for allografting. Progenitor cells can be effectively mobilized into peripheral blood in majority of healthy donors with a brief administration of G-CSF. A mobilization course in 111 donors (median age 40years) was retrospectively studied and the factors influencing the efficacy of mobilization were analyzed. The median number of CD34+ cells per kg recipient weight 5.1x10(6) was obtained after a median of two aphereses. The target cell dose (4.0x10(6)/kg) was reached in 69% of donors. Circulating CD34+ count and CD34+ yield were negatively associated with donor's age. Other independent factors associated with superior yield were precollection platelet and WBC counts. In multivariate analysis only CD34+ precount predicted for CD34+ yield. G-CSF had an acceptable short-term safety profile. Our data confirm that apheresis is a safe procedure in healthy including aged donors and suggest that older donors could be poorer mobilizers than younger.