Syngeneic (A) and allogeneic (B) bone marrow engraftment after pretreatment with BU with or without CY. Male C57BL/6JIco (B6-Gpi-1^b/Gpi-1^b) mice (Iffa Credo, L’Arbresle, France), 12 to 16 weeks old and weighing 25 to 30 g, were used as recipients. Congenic C57BL/6J-Gpi-1^a/Gpi-1^a(B6-Gpi-1^a) and BALB.B10 mice (Jackson Laboratory, Bar Harbour, ME) were used as the source of syngeneic and H-2 compatible allogeneic donor bone marrow, respectively. Busulfan was injected intraperitoneally (IP) as a suspension in corn oil as fractionated doses (4 × 25 mg/kg) on 4 consecutive days. CY was administered (IP in phosphate-buffered saline, 200 mg/kg) 24 hours after (the last dose of) BU. BMT (10⁶ bone marrow cells) was performed 24 hours after the last drug treatment or 48 hours after BU when CY was not in the regimen. Shown are the means (±SD, 4 to 6 mice per group) as percentages of Gpi-1^a type erythroid chimerism up to 5 months after BMT. Asterisks indicate significance in Mann-Whitney-U test (*P < .01; **P < .05).