The present study provides the first clear in vitro evidence to suggest that CBMCs are less able than PBMCs to induce skin GVH type alloreactivity in HLA-mismatched pairs. The severity of in vitro GVH type alloreactivity (graded as I-IV) was strongly associated with the levels of alloreactive CTLp frequencies. The low cellular alloreactivity of CBMCs detected in vitro suggests that in a proportion of cases HLA-mismatched unrelated CB may not give rise to severe GVHD in vivo after transplantation.