Peripheral stem cells obtained by stimulation of haematopoiesis by administration of so-called growth factors are used for allogenic transplantations of haematopoietic cells only since the mid-nineties of the 20th century. So far however not all potential undesirable effects of growth factor administration to healthy subjects are known. The authors present the results of a programme of allogene transplantation of peripheral blood stem cells from related donors which was started in their department in 1996. From 11/1996 till 8/2000 the authors started 55 mobilizations of peripheral stem cells in donors before transplantation. The mobilization growth factor G-CSF was used for in a total dose of 16 micrograms/kg/day s.c. administered in two daily doses. During administration of G-CSF 19 donors had no complaints, 34 donors (64%) had pain in their bones, subfebrile temperatures and/or symptoms of flu. G-CSF produced dramatic changes in the haemogram--massive leucocytosis with dominant neutrophilia, after collection of the transplant thrombocytopenia developed. The changes of some biochemical parameters were also marked. A peripheral venous approach for collection of the transplant was applied in 23 instances (43%), in the remainder cannulation of a central vein was used. A total of 121 leukaophereses was performed in 53 donors (separator COBE Spectra) which were started on the 4th day of G-CSF administration. An automatic operation of PBSC collection was used in 28 leukaphereses, an operation of mononuclear collection in 93 leukaphereses. In leukaphereses only mild symptoms of hypocalcaemia were observed in 26 donors (48%). The mean total yield of leukaphereses was 9.39 x 10(6) CD34+ cells/kg of the recipient (3.48-23.13) and 76.54 x 10(4) CFU-GM/kg of the recipient (10.02-238.78), no mobilization failed. Of 36 donors 25 (69%) came for a check-up one year after collection of the transplant. No changes in the haemogram nor in biochemical parameters were recorded. In donors with hepatopathies pathological findings persisted in biochemical examinations without a deteriorating trend. Administration of G-CSF to healthy subjects is a new technology associated with certain advantages as well as some questions. Safety of donors holds the first place, detailed information is essential as well as examination and follow up not only on a short-term but also on a long-term basis.