Thirty one peripheral blood stem cell mobilizations using cyclophosphamide, followed by leucapheresis, were performed in 25 patients with a variety of haematological malignancies. Cyclophosphamide at doses ranging from 0.25 g/M2/day x 4 days to 4 g/M2 on one day were given. Total doses of cyclophosphamide less than 4 g/M2 failed to induce a significant mobilization of stem cells into the peripheral blood. This compares to cyclophosphamide doses of 4 g/M2 given over one or two days where 70%-75% of patients yielded adequate stem cells for transplantation (greater than 20 x 10(4) CFU-GM/kg). Both the presence of tumour cells in the bone marrow and previous chemotherapy within 12 months of cyclophosphamide infusion significantly diminished the patients stem cell mobilizations. The rate of recovery of the patient's leucocyte count following cyclophosphamide was highly correlated to the peak level of PB CFU-GM and is a good indicator of the total stem cell yield. Fever in 50% of patients during the period of cytopaenia was the only complication seen in our patients and thus high dose cyclophosphamide is a suitable and safe agent for mobilizing haemopoietic stem cells.