Bone marrow was harvested from a 3.95 kg premature 7-week-old female baby for donation to a 13 kg HLA-identical sister with severe aplastic anemia. Two hundred ml of donor bone marrow were aspirated, containing a calculated dose of 3 x 10(8)/kg nucleated bone marrow cells for the recipient. This was equivalent to two-thirds of the donor's calculated blood volume (320 ml). Peri-operative care included invasive monitoring of intravascular pressures, arterial blood gas analysis, careful temperature control and the infusion of 150 ml of packed red cells, 150 ml of colloid and 50 ml of crystalloid. Rapid engraftment occurred. There were no complications and both donor and recipient are healthy 12 months later.