Twenty-seven patients undergoing matched sibling BMT were randomly assigned to be infused with bone marrow alone or bone marrow supplemented with allogeneic peripheral blood cells collected by apheresis after stimulation with filgrastim. Other transplant conditions were standard and identical for the two groups. There was no difference between the groups in survival or acute or chronic GVHD, however, the patients receiving blood cells had significantly more rapid neutrophil engraftment by a median of 2 days. We conclude that filgrastim-mobilised HLA-identical sibling allogeneic blood cells are biologically active and safe.