Clinically detectable splenomegaly and splenic rupture are uncommon but potentially life-threatening consequences of G-CSF administration. Increased spleen size in mice injected with G-CSF is a complex genetic trait amenable to investigation in experimental inter-strain crosses by quantitative trait analysis. A quantitative trait locus (QTL) with highly significant linkage (LOD 7.9) for splenomegaly was identified within a 22 centimorgan (cM) region on chromosome 1. Inheritance of a C57BL/6 haplotype in this region was associated with a greater spleen weight. The relevance of this locus was confirmed by analysing the responses of mice congenic for the distal 12 cM of this region (C57BL/6 and C57BL/6.SJL-Ptprc(a) Pep3(b)). Consistent with the QTL effect, mice lacking C57BL/6 alleles in this region had reduced splenomegaly induced by G-CSF. Intriguingly, peripheral blood neutrophilia and progenitor cell mobilisation responses to G-CSF were also significantly influenced.