Stem cell mobilisation can be achieved either by administration of rhG-CSF alone or after high-dose cyclophosphamide (HDCy) plus rhG-CSF. We have compared both mobilisation procedures intra-individually in 43 patients with haematological malignancies. Furthermore, the toxicity data were registered. The CD34+ cell yield was higher after mobilisation with HDCy plus rhG-CSF than after rhG-CSF alone in 21 out of 22 patients who were actually harvested after both procedures. If a patient mobilised insufficiently after rhG-CSF alone, the yield of CD34+ cells after the following HDCy priming was lower compared to patients who mobilised sufficiently after rhG-CSF priming alone. In 12 patients with B cell malignancies a reduced number of B cells such as CD10+, CD19+, CD20+ cells in bone marrow as well as in leukapheresis products was observed after HDCy plus rhG-CSF compared to rhG-CSF alone. Toxicity data revealed HDCy as a relatively toxic priming regimen with all patients hospitalised and 74% experiencing neutropenic fever and administration of intravenous antibiotics. In two patients, seizure-like episodes were observed during cyclophosphamide bolus infusion. In conclusion, HDCy increased the yield of CD34+cell and reduced B cells in leukapheresis products indicating reduced tumour cell load compared with rhG-CSF priming alone. The efficacy of HDCy priming is limited by its profound toxicity and morbidity. Studies evaluating efficacy and safety of lower doses of cyclophosphamide are needed. Bone Marrow Transplantation (2000) 26, 717-722.