Treating malignant disease with very high dose chemotherapy, often with total body irradiation, has become established during the past decade, particularly for lymphomas and leukaemias. Reinfusion of some of the patient's bone marrow (collected before high dose chemotherapy) has improved the outcome of these very intensive regimens. These autologous bone marrow transplants (autografts) entail about four weeks' pancytopenia,1 and despite improvements in supportive care this procedure is associated with substantial morbidity and a 5–10% mortality, mainly due to myelosuppression.

Normal bone marrow contains enough undifferentiated stem cells to allow long term reconstitution (engraftment), but it seems to lack those more committed progenitors that would lead to engraftment within two or three weeks. In the past few years it has become possible to collect such committed progenitor and stem cells not from wihin the bone marrow cavity but from peripheral blood. The use of peripheral blood stem cells as autografts has changed much of haematological practice and will transform medical oncology within the next few years.

Until 1984, the stem cell content of either bone marrow or peripheral blood could be estimated only indirectly by in vitro colony forming assays. Since then a monoclonal antibody, now …