Peripheral blood stem cells (PBSC) harvest may be difficult in young children. Extracorporeal separator line priming by red blood cells is usually required to improve haemodynamic tolerance and efficacy of collection. We present our experience with 24 children weighing less than 15 kg treated between January 1997 and September 1999, in whom we tried to avoid systematic blood priming. The median age and weight at the time of apheresis were 2.4 years and 12 kg, respectively. A total of 48 PBSC were performed. When haemoglobin was less than 12 g/dl, packed red cells were transfused before collection (40% of aphereses). The median cell yield per apheresis was 7.1 (2.2-30.6)x10(6)/kg CD34(+) cells and 16.0 (3.3-44.3)x10(5) CFU-GM/kg. Initial collection failed in three cases. Four children required an additional haematopoietic progenitor mobilization. This procedure allowed PBSC collection without transfusion in 37.5% of children, and was safe (two serious and five mild transient side effects) and effective (median CD34(+) cells collected per child: 7.1 x 10(6)/kg (4.6-30.6) and CFU-GM: 15.1 x 10(5)/kg (4.7-44.3)). Despite their low weight, insertion of a femoral catheter was avoided in 43% of children.